cc:: Change plumbing for external_begin_frame_source to the Scheduler.

cc/trees/single_thread_proxy.cc

Index: cc/trees/single_thread_proxy.cc
diff --git a/cc/trees/single_thread_proxy.cc b/cc/trees/single_thread_proxy.cc
index c8e5829b22f1538d43c0c2f9c7db751f07a7d87a..cb669abdf10791e58bc64293a704c216fe8a2bb2 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -25,22 +25,17 @@ namespace cc {
 scoped_ptr<Proxy> SingleThreadProxy::Create(
     LayerTreeHost* layer_tree_host,
     LayerTreeHostSingleThreadClient* client,
-    TaskRunnerProvider* task_runner_provider,
-    scoped_ptr<BeginFrameSource> external_begin_frame_source) {
+    TaskRunnerProvider* task_runner_provider) {
   return make_scoped_ptr(
-      new SingleThreadProxy(layer_tree_host, client, task_runner_provider,
-                            std::move(external_begin_frame_source)));
+      new SingleThreadProxy(layer_tree_host, client, task_runner_provider));
 }
 
-SingleThreadProxy::SingleThreadProxy(
-    LayerTreeHost* layer_tree_host,
-    LayerTreeHostSingleThreadClient* client,
-    TaskRunnerProvider* task_runner_provider,
-    scoped_ptr<BeginFrameSource> external_begin_frame_source)
+SingleThreadProxy::SingleThreadProxy(LayerTreeHost* layer_tree_host,
+                                     LayerTreeHostSingleThreadClient* client,
+                                     TaskRunnerProvider* task_runner_provider)
     : layer_tree_host_(layer_tree_host),
       client_(client),
       task_runner_provider_(task_runner_provider),
-      external_begin_frame_source_(std::move(external_begin_frame_source)),
       next_frame_is_newly_committed_frame_(false),
 #if DCHECK_IS_ON()
       inside_impl_frame_(false),
@@ -56,17 +51,23 @@ SingleThreadProxy::SingleThreadProxy(
   DCHECK(task_runner_provider_);
   DCHECK(task_runner_provider_->IsMainThread());
   DCHECK(layer_tree_host);
+}
 
-  if (layer_tree_host->settings().single_thread_proxy_scheduler &&
+void SingleThreadProxy::Start(
+    scoped_ptr<BeginFrameSource> external_begin_frame_source) {
+  DebugScopedSetImplThread impl(task_runner_provider_);
+  external_begin_frame_source_ = std::move(external_begin_frame_source);
+
+  if (layer_tree_host_->settings().single_thread_proxy_scheduler &&
       !scheduler_on_impl_thread_) {
     SchedulerSettings scheduler_settings(
-        layer_tree_host->settings().ToSchedulerSettings());
+        layer_tree_host_->settings().ToSchedulerSettings());
     scheduler_settings.commit_to_active_tree = CommitToActiveTree();
 
     scoped_ptr<CompositorTimingHistory> compositor_timing_history(
         new CompositorTimingHistory(
             CompositorTimingHistory::BROWSER_UMA,
-            layer_tree_host->rendering_stats_instrumentation()));
+            layer_tree_host_->rendering_stats_instrumentation()));
 
     scheduler_on_impl_thread_ =
         Scheduler::Create(this, scheduler_settings, layer_tree_host_->id(),
@@ -74,10 +75,7 @@ SingleThreadProxy::SingleThreadProxy(
                           external_begin_frame_source_.get(),
                           std::move(compositor_timing_history));
   }
-}
 
-void SingleThreadProxy::Start() {
-  DebugScopedSetImplThread impl(task_runner_provider_);
   layer_tree_host_impl_ = layer_tree_host_->CreateLayerTreeHostImpl(this);
 }