cc: Remove BeginFrameSourcesConstructor.

cc/trees/thread_proxy.cc

Index: cc/trees/thread_proxy.cc
diff --git a/cc/trees/thread_proxy.cc b/cc/trees/thread_proxy.cc
index 94e8d89b7118cdbb4deec6686726189c51e08bba..bca8a8c6a89381b663fb6f3d650f8e9a3b1e4e02 100644
--- a/cc/trees/thread_proxy.cc
+++ b/cc/trees/thread_proxy.cc
@@ -1203,11 +1203,8 @@ void ThreadProxy::InitializeImplOnImplThread(CompletionEvent* completion) {
   SchedulerSettings scheduler_settings(
       layer_tree_host()->settings().ToSchedulerSettings());
   impl().scheduler = Scheduler::Create(
-                         this,
-                         scheduler_settings,
-                         impl().layer_tree_host_id,
-                         ImplThreadTaskRunner(),
-                         impl().external_begin_frame_source.Pass());
+      this, scheduler_settings, impl().layer_tree_host_id,
+      ImplThreadTaskRunner(), impl().external_begin_frame_source.get());
   impl().scheduler->SetVisible(impl().layer_tree_host_impl->visible());
   impl_thread_weak_ptr_ = impl().weak_factory.GetWeakPtr();
   completion->Signal();
@@ -1267,6 +1264,7 @@ void ThreadProxy::LayerTreeHostClosedOnImplThread(CompletionEvent* completion) {
       impl().layer_tree_host_impl->resource_provider());
   impl().current_resource_update_controller = nullptr;
   impl().scheduler = nullptr;
+  impl().external_begin_frame_source = nullptr;
   impl().layer_tree_host_impl = nullptr;
   impl().weak_factory.InvalidateWeakPtrs();
   // We need to explicitly shutdown the notifier to destroy any weakptrs it is