WebView: Ensure AwContentBrowserClient::HandleExternalProtocol is called on UI thread

Before this CL, AwContentBrowserClient::HandleExternalProtocol() assumed
that it can be called from the IO thread, but actually this function is
always called on the UI thread. This CL removes the unnecessary thread
check from the function.

Note that the class-level comment of ContentBrowserClient says: "The
methods are assumed to be called on the UI thread unless otherwise
specified.". The function-level comment of
ContentBrowserClient::HandleExternalProtocol() doesn't mention the
thread affinity, so it should be valid to assume that this function is
always called on the UI thread.

Bug: 373474043
Change-Id: Ie087abc4380a281335808902c5b1f075fae260ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5933488
Reviewed-by: Richard (Torne) Coles <torne@chromium.org>
Commit-Queue: Richard (Torne) Coles <torne@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Auto-Submit: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1373324}

android_webview/browser/aw_content_browser_client.cc

@@ -913,6 +913,8 @@ bool AwContentBrowserClient::HandleExternalProtocol(
     content::RenderFrameHost* initiator_document,
     const net::IsolationInfo& isolation_info,
     mojo::PendingRemote<network::mojom::URLLoaderFactory>* out_factory) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
   // Sandbox flags
   // =============
   //
@@ -947,39 +949,28 @@ bool AwContentBrowserClient::HandleExternalProtocol(
   // be schemes unrelated to the regular network stack so it doesn't make sense
   // to look for cookies. Providing a nullopt for the cookie manager lets
   // the AwProxyingURLLoaderFactory know to skip that work.
-  if (content::BrowserThread::CurrentlyOn(content::BrowserThread::IO)) {
-    // Manages its own lifetime.
-    new android_webview::AwProxyingURLLoaderFactory(
-        std::nullopt /* cookie_manager */, nullptr /* cookie_access_policy */,
-        isolation_info, web_contents_key, frame_tree_node_id,
-        std::move(receiver), mojo::NullRemote(), true /* intercept_only */,
-        std::nullopt /* security_options */,
-        nullptr /* xrw_allowlist_matcher */, std::move(browser_context_handle),
-        std::nullopt /* navigation_id */);
-  } else {
-    content::GetIOThreadTaskRunner({})->PostTask(
-        FROM_HERE,
-        base::BindOnce(
-            [](mojo::PendingReceiver<network::mojom::URLLoaderFactory> receiver,
-               std::optional<WebContentsKey> web_contents_key,
-               content::FrameTreeNodeId frame_tree_node_id,
-               scoped_refptr<AwBrowserContextIoThreadHandle>
-                   browser_context_handle,
-               const net::IsolationInfo& isolation_info) {
-              // Manages its own lifetime.
-              new android_webview::AwProxyingURLLoaderFactory(
-                  std::nullopt /* cookie_manager */,
-                  nullptr /* cookie_access_policy */, isolation_info,
-                  web_contents_key, frame_tree_node_id, std::move(receiver),
-                  mojo::NullRemote(), true /* intercept_only */,
-                  std::nullopt /* security_options */,
-                  nullptr /* xrw_allowlist_matcher */,
-                  std::move(browser_context_handle),
-                  std::nullopt /* navigation_id */);
-            },
-            std::move(receiver), web_contents_key, frame_tree_node_id,
-            std::move(browser_context_handle), isolation_info));
-  }
+  content::GetIOThreadTaskRunner({})->PostTask(
+      FROM_HERE,
+      base::BindOnce(
+          [](mojo::PendingReceiver<network::mojom::URLLoaderFactory> receiver,
+             std::optional<WebContentsKey> web_contents_key,
+             content::FrameTreeNodeId frame_tree_node_id,
+             scoped_refptr<AwBrowserContextIoThreadHandle>
+                 browser_context_handle,
+             const net::IsolationInfo& isolation_info) {
+            // Manages its own lifetime.
+            new android_webview::AwProxyingURLLoaderFactory(
+                std::nullopt /* cookie_manager */,
+                nullptr /* cookie_access_policy */, isolation_info,
+                web_contents_key, frame_tree_node_id, std::move(receiver),
+                mojo::NullRemote(), true /* intercept_only */,
+                std::nullopt /* security_options */,
+                nullptr /* xrw_allowlist_matcher */,
+                std::move(browser_context_handle),
+                std::nullopt /* navigation_id */);
+          },
+          std::move(receiver), web_contents_key, frame_tree_node_id,
+          std::move(browser_context_handle), isolation_info));
   return false;
 }