Add new CastWebContents::Observer callback

This CL adds a new callback to the public implementation of
CastWebContents. This is used to allow clients to hook in to when
navigation is ready, which is needed for including mirroring in the
Cast Runtime.

For comparison with eqivalent fuchsia code, see:
https://source.chromium.org/chromium/chromium/src/+/main:fuchsia/engine/browser/frame_impl.cc;l=1161

Bug: internal b/188547579
Change-Id: I5bb83e021b6073e965bdada7769a1cdcdec0fb61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2915660
Reviewed-by: Sean Topping <seantopping@chromium.org>
Commit-Queue: Ryan Keane <rwkeane@google.com>
Cr-Commit-Position: refs/heads/master@{#886335}

chromecast/browser/cast_web_contents.h

@@ -27,6 +27,7 @@ class AssociatedInterfaceProvider;
 }  // namespace blink
 
 namespace content {
+class NavigationHandle;
 class WebContents;
 }  // namespace content
 
@@ -155,6 +156,11 @@ class CastWebContents {
         service_manager::InterfaceProvider* frame_interfaces,
         blink::AssociatedInterfaceProvider* frame_associated_interfaces) {}
 
+    // Called when the navigation is ready to be committed in the WebContents'
+    // main frame.
+    virtual void MainFrameReadyToCommitNavigation(
+        content::NavigationHandle* navigation_handle) {}
+
     // A navigation has finished in the WebContents' main frame.
     virtual void MainFrameFinishedNavigation() {}
 

chromecast/browser/cast_web_contents_impl.cc

@@ -643,14 +643,17 @@ void CastWebContentsImpl::ReadyToCommitNavigation(
   // Skip injecting bindings scripts if |navigation_handle| is not
   // 'current' main frame navigation, e.g. another DidStartNavigation is
   // emitted. Also skip injecting for same document navigation and error page.
-  if (navigation_handle != active_navigation_ ||
-      navigation_handle->IsErrorPage()) {
-    return;
+  if (navigation_handle == active_navigation_ &&
+      !navigation_handle->IsErrorPage()) {
+    // Injects registered bindings script into the main frame.
+    script_injector_.InjectScriptsForURL(
+        navigation_handle->GetURL(), navigation_handle->GetRenderFrameHost());
   }
 
-  // Injects registered bindings script into the main frame.
-  script_injector_.InjectScriptsForURL(navigation_handle->GetURL(),
-                                       navigation_handle->GetRenderFrameHost());
+  // Notifies observers that the navigation of the main frame is ready.
+  for (Observer& observer : observer_list_) {
+    observer.MainFrameReadyToCommitNavigation(navigation_handle);
+  }
 }
 
 void CastWebContentsImpl::DidFinishNavigation(