CrOS: Enable threaded UI compositor by default.

ui/compositor/compositor.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/compositor/compositor.h"
 
 #include <algorithm>
 #include <deque>
 
 #include "base/bind.h"
@@ skipping 517 matching lines @@
     root_layer_->SetCompositor(NULL);
 
   // Stop all outstanding draws before telling the ContextFactory to tear
   // down any contexts that the |host_| may rely upon.
   host_.reset();
 
   ContextFactory::GetInstance()->RemoveCompositor(this);
 }
 
 // static
-void Compositor::Initialize(bool use_thread) {
+void Compositor::Initialize() {
+#if defined(OS_CHROMEOS)
+  bool use_thread = !CommandLine::ForCurrentProcess()->HasSwitch(
+      switches::kUIDisableThreadedCompositing);
+#else
+  bool use_thread =
+      CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kUIEnableThreadedCompositing) &&
+      !CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kUIDisableThreadedCompositing);
+#endif
   if (use_thread) {
     g_compositor_thread = new base::Thread("Browser Compositor");
     g_compositor_thread->Start();
   }
 }
 
 // static
 bool Compositor::WasInitializedWithThread() {
   return !!g_compositor_thread;
 }
@@ skipping 261 matching lines @@
 COMPOSITOR_EXPORT void DisableTestCompositor() {
   ResetImplicitFactory();
   g_test_compositor_enabled = false;
 }
 
 COMPOSITOR_EXPORT bool IsTestCompositorEnabled() {
   return g_test_compositor_enabled;
 }
 
 }  // namespace ui