cc: Remove BeginFrameSourcesConstructor.

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 91816b9c057daff0b81c3f6b2fb1153cfdab537b..8e84b8a457f0358476ee370326f0ab80ef4f7adb 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -42,6 +42,7 @@ SingleThreadProxy::SingleThreadProxy(
     : Proxy(main_task_runner, NULL),
       layer_tree_host_(layer_tree_host),
       client_(client),
+      external_begin_frame_source_(external_begin_frame_source.Pass()),
       timing_history_(layer_tree_host->rendering_stats_instrumentation()),
       next_frame_is_newly_committed_frame_(false),
 #if DCHECK_IS_ON()
@@ -66,7 +67,7 @@ SingleThreadProxy::SingleThreadProxy(
     scheduler_settings.main_thread_should_always_be_low_latency = true;
     scheduler_on_impl_thread_ = Scheduler::Create(
         this, scheduler_settings, layer_tree_host_->id(),
-        MainThreadTaskRunner(), external_begin_frame_source.Pass());
+        MainThreadTaskRunner(), external_begin_frame_source_.get());
   }
 }