webview: Replace LITTLE-only cmdline flag with base::Feature

Makes it possible to enable LITTLE-cores-only mode via Finch, by
replacing the existing command line flag with a base::Feature.

It can still be enabled locally by passing
--enable-features=WebViewCpuAffinityRestrictToLittleCores or
enabling the feature in WebView DevTools.

Test: Run WV shell with flag above & check chrome://histograms/Power.
Bug: 1111789
Change-Id: I209b132ccd5354e41849a33bb0509773b619fc62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2502511
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821685}

android_webview/browser/aw_content_browser_client.cc

@@ -421,11 +421,6 @@ void AwContentBrowserClient::AppendExtraCommandLineSwitches(
             ::switches::kEnableCrashReporterForTesting)) {
       command_line->AppendSwitch(::switches::kEnableCrashReporterForTesting);
     }
-    // Pass WebView's force little cores flag to renderer process.
-    if (base::CommandLine::ForCurrentProcess()->HasSwitch(
-            ::switches::kWebViewForceLittleCores)) {
-      command_line->AppendSwitch(::switches::kWebViewForceLittleCores);
-    }
   }
 }
 

android_webview/common/aw_features.cc

@@ -18,6 +18,12 @@ const base::Feature kWebViewBrotliSupport{"WebViewBrotliSupport",
 const base::Feature kWebViewConnectionlessSafeBrowsing{
     "WebViewConnectionlessSafeBrowsing", base::FEATURE_DISABLED_BY_DEFAULT};
 
+// Restricts WebView child processes to use only LITTLE cores on big.LITTLE
+// architectures.
+const base::Feature kWebViewCpuAffinityRestrictToLittleCores{
+    "WebViewCpuAffinityRestrictToLittleCores",
+    base::FEATURE_DISABLED_BY_DEFAULT};
+
 // Enable display cutout support for Android P and above.
 const base::Feature kWebViewDisplayCutout{"WebViewDisplayCutout",
                                           base::FEATURE_DISABLED_BY_DEFAULT};

android_webview/common/aw_features.h

@@ -16,6 +16,7 @@ namespace features {
 // Alphabetical:
 extern const base::Feature kWebViewBrotliSupport;
 extern const base::Feature kWebViewConnectionlessSafeBrowsing;
+extern const base::Feature kWebViewCpuAffinityRestrictToLittleCores;
 extern const base::Feature kWebViewDisplayCutout;
 extern const base::Feature kWebViewExtraHeadersSameDomainOnly;
 extern const base::Feature kWebViewExtraHeadersSameOriginOnly;

android_webview/common/aw_switches.cc

@@ -46,7 +46,4 @@ const char kFinchSeedMinDownloadPeriod[] = "finch-seed-min-download-period";
 // variations seed.
 const char kFinchSeedMinUpdatePeriod[] = "finch-seed-min-update-period";
 
-// Force WebView renderer to run in little cores
-const char kWebViewForceLittleCores[] = "webview-force-little-cores";
-
 }  // namespace switches

android_webview/common/aw_switches.h

@@ -17,7 +17,6 @@ extern const char kFinchSeedExpirationAge[];
 extern const char kFinchSeedIgnorePendingDownload[];
 extern const char kFinchSeedMinDownloadPeriod[];
 extern const char kFinchSeedMinUpdatePeriod[];
-extern const char kWebViewForceLittleCores[];
 
 }  // namespace switches
 

android_webview/java/src/org/chromium/android_webview/common/ProductionSupportedFlagList.java

@@ -96,7 +96,7 @@ public final class ProductionSupportedFlagList {
                             + "Elements v0, and HTML Imports)."),
             Flag.baseFeature(AwFeatures.WEBVIEW_DISPLAY_CUTOUT,
                     "Enables display cutout (notch) support in WebView for Android P and above."),
-            Flag.commandLine(AwSwitches.WEBVIEW_FORCE_LITTLE_CORES,
+            Flag.baseFeature(AwFeatures.WEBVIEW_CPU_AFFINITY_RESTRICT_TO_LITTLE_CORES,
                     "Forces WebView to do rendering work in little cores"),
             Flag.baseFeature(BlinkFeatures.WEBVIEW_ACCELERATE_SMALL_CANVASES,
                     "Accelerate all canvases in webview."),

android_webview/lib/aw_main_delegate.cc

@@ -299,10 +299,6 @@ void AwMainDelegate::PreSandboxStartup() {
 
   if (process_type == switches::kRendererProcess) {
     InitResourceBundleRendererSide();
-    if (command_line.HasSwitch(switches::kWebViewForceLittleCores)) {
-      content::EnforceProcessCpuAffinity(
-          base::CpuAffinityMode::kLittleCoresOnly);
-    }
   }
 
   EnableCrashReporter(process_type);
@@ -373,6 +369,15 @@ void AwMainDelegate::PostFieldTrialInitialization() {
   ALLOW_UNUSED_LOCAL(is_canary_dev);
   ALLOW_UNUSED_LOCAL(is_browser_process);
 
+  // Enable LITTLE-cores only mode if the feature is enabled, but only for child
+  // processes, as the browser process is shared with the hosting app.
+  if (!is_browser_process &&
+      base::FeatureList::IsEnabled(
+          android_webview::features::
+              kWebViewCpuAffinityRestrictToLittleCores)) {
+    content::EnforceProcessCpuAffinity(base::CpuAffinityMode::kLittleCoresOnly);
+  }
+
 #if BUILDFLAG(ENABLE_GWP_ASAN_MALLOC)
   gwp_asan::EnableForMalloc(is_canary_dev || is_browser_process,
                             process_type.c_str());