Add threaded compositing mode to field trial

content/renderer/render_thread_impl.cc

Index: content/renderer/render_thread_impl.cc
diff --git a/content/renderer/render_thread_impl.cc b/content/renderer/render_thread_impl.cc
index 0cb91368ccc5c37f0bc30a9faf469f5268b5da20..28b4a58b3925aba273db3cbbec3bfebb11f1278b 100644
--- a/content/renderer/render_thread_impl.cc
+++ b/content/renderer/render_thread_impl.cc
@@ -40,6 +40,7 @@
 #include "content/common/resource_messages.h"
 #include "content/common/view_messages.h"
 #include "content/common/web_database_observer_impl.h"
+#include "content/public/common/content_constants.h"
 #include "content/public/common/content_paths.h"
 #include "content/public/common/content_switches.h"
 #include "content/public/common/renderer_preferences.h"
@@ -492,6 +493,10 @@ void RenderThreadImpl::EnsureWebKitInitialized() {
   webkit_platform_support_.reset(new RendererWebKitPlatformSupportImpl);
   WebKit::initialize(webkit_platform_support_.get());
 
+  base::FieldTrial* thread_trial =
+      base::FieldTrialList::Find(content::kGpuCompositingFieldTrialName);
+  bool is_thread_trial = thread_trial && thread_trial->group_name() ==
+      content::kGpuCompositingFieldTrialThreadEnabledName;
   bool has_enable = CommandLine::ForCurrentProcess()->HasSwitch(
       switches::kEnableThreadedCompositing);
   bool has_disable = CommandLine::ForCurrentProcess()->HasSwitch(
@@ -501,13 +506,15 @@ void RenderThreadImpl::EnsureWebKitInitialized() {
   // The new design can be tracked at: http://crbug.com/134492.
   bool is_guest = CommandLine::ForCurrentProcess()->HasSwitch(
       switches::kGuestRenderer);
-  bool enable = has_enable && (!has_disable) && (!is_guest);
+  DCHECK(!is_thread_trial || !has_disable);
+  bool enable = (is_thread_trial || (has_enable && !has_disable)) && !is_guest;
   if (enable) {
     compositor_thread_.reset(new CompositorThread(this));
     AddFilter(compositor_thread_->GetMessageFilter());
     WebKit::WebCompositor::initialize(compositor_thread_->GetWebThread());
-  } else
+  } else {
     WebKit::WebCompositor::initialize(NULL);
+  }
   compositor_initialized_ = true;
 
   WebScriptController::enableV8SingleThreadMode();