Add a feature parameter to fork the prefetch process from the renderer

Advantages of forking from the renderer instead of the browser:
- Prioritizes prefetching the native library used by the renderer, which is more critical for page load latency.
- Leverages the native library shared with Chrome, ensuring it is always ordered.
- Reduces the browser process's workload during WebView startup.

Potential drawback:
- If the renderer initialization is delayed, the prefetch operation may also be delayed.

Bug: 395567815
Change-Id: I502ccfe243bbe4f34921468a50b93bf9dc97222c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6299173
Commit-Queue: Ziad Youssef <ziadyoussef@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1425084}

android_webview/common/aw_features.cc

@@ -207,6 +207,10 @@ BASE_FEATURE(kWebViewPrefetchNativeLibrary,
              "WebViewPrefetchNativeLibrary",
              base::FEATURE_DISABLED_BY_DEFAULT);
 
+// A parameter to trigger the prefetch from the renderer instead of the browser.
+const base::FeatureParam<bool> kWebViewPrefetchFromRenderer{
+    &kWebViewPrefetchNativeLibrary, "WebViewPrefetchFromRenderer", false};
+
 // If enabled TYPE_SCROLLED accessibility events are sent every 100ms when user
 // is scrolling irrespective of GestureScrollUpdate being consumed or not.
 // If disabled events are sent on GSU consumed ack.

android_webview/common/aw_features.h

@@ -47,6 +47,7 @@ BASE_DECLARE_FEATURE(kWebViewReduceUAAndroidVersionDeviceModel);
 BASE_DECLARE_FEATURE(kWebViewEnableCrash);
 BASE_DECLARE_FEATURE(kWebViewPreloadClasses);
 BASE_DECLARE_FEATURE(kWebViewPrefetchNativeLibrary);
+extern const base::FeatureParam<bool> kWebViewPrefetchFromRenderer;
 BASE_DECLARE_FEATURE(kWebViewSeparateResourceContext);
 BASE_DECLARE_FEATURE(kWebViewDoNotSendAccessibilityEventsOnGSU);
 BASE_DECLARE_FEATURE(kWebViewHyperlinkContextMenu);

android_webview/glue/java/src/com/android/webview/chromium/WebViewChromiumAwInit.java

@@ -399,7 +399,12 @@ public class WebViewChromiumAwInit {
                                 AwFeatureMap.isEnabled(AwFeatures.WEBVIEW_DISABLE_CHIPS));
                     });
 
-            if (AwFeatureMap.isEnabled(AwFeatures.WEBVIEW_PREFETCH_NATIVE_LIBRARY)) {
+            if (AwFeatureMap.isEnabled(AwFeatures.WEBVIEW_PREFETCH_NATIVE_LIBRARY)
+                    && !AwFeatureMap.getInstance()
+                            .getFieldTrialParamByFeatureAsBoolean(
+                                    AwFeatures.WEBVIEW_PREFETCH_NATIVE_LIBRARY,
+                                    "WebViewPrefetchFromRenderer",
+                                    false)) {
                 PostTask.postTask(
                         TaskTraits.BEST_EFFORT,
                         () -> {

android_webview/renderer/aw_content_renderer_client.cc

@@ -18,10 +18,12 @@
 #include "android_webview/renderer/aw_render_view_ext.h"
 #include "android_webview/renderer/aw_url_loader_throttle_provider.h"
 #include "android_webview/renderer/browser_exposed_renderer_interfaces.h"
+#include "base/android/library_loader/library_prefetcher.h"
 #include "base/command_line.h"
 #include "base/i18n/rtl.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/strings/string_util.h"
+#include "base/task/thread_pool.h"
 #include "components/android_system_error_page/error_page_populator.h"
 #include "components/autofill/core/common/autofill_features.h"
 #include "components/cdm/renderer/key_system_support_update.h"
@@ -73,6 +75,15 @@ void AwContentRendererClient::RenderThreadStarted() {
   browser_interface_broker_ =
       blink::Platform::Current()->GetBrowserInterfaceBroker();
 
+  if (base::FeatureList::IsEnabled(features::kWebViewPrefetchNativeLibrary) &&
+      features::kWebViewPrefetchFromRenderer.Get()) {
+    base::ThreadPool::PostTask(
+        FROM_HERE, base::BindOnce([] {
+          base::android::NativeLibraryPrefetcher::ForkAndPrefetchNativeLibrary(
+              false);
+        }));
+  }
+
 #if BUILDFLAG(ENABLE_SPELLCHECK)
   if (!spellcheck_)
     spellcheck_ = std::make_unique<SpellCheck>(this);

testing/variations/fieldtrial_testing_config.json

@@ -26352,6 +26352,9 @@
             "experiments": [
                 {
                     "name": "Enabled",
+                    "params": {
+                        "WebViewPrefetchFromRenderer": "true"
+                    },
                     "enable_features": [
                         "WebViewPrefetchNativeLibrary"
                     ]