Make SingleThreadProxy a SchedulerClient

cc/scheduler/scheduler.cc

Index: cc/scheduler/scheduler.cc
diff --git a/cc/scheduler/scheduler.cc b/cc/scheduler/scheduler.cc
index 4a682cdb4418f973bbae6475caf8831f4688ab2e..a4547fade313dfb032b40ec191b09b11b4fd6f29 100644
--- a/cc/scheduler/scheduler.cc
+++ b/cc/scheduler/scheduler.cc
@@ -272,6 +272,9 @@ base::TimeTicks Scheduler::LastBeginImplFrameTime() {
 }
 
 void Scheduler::SetupNextBeginFrameIfNeeded() {
+  if (!impl_task_runner_)
+    return;
+
   bool needs_begin_frame = state_machine_.BeginFrameNeeded();
 
   if (settings_.throttle_frame_production) {
@@ -497,8 +500,8 @@ void Scheduler::PostBeginRetroFrameIfNeeded() {
 // any asynchronous animation and scroll/pinch updates.
 void Scheduler::BeginImplFrame(const BeginFrameArgs& args) {
   TRACE_EVENT1("cc", "Scheduler::BeginImplFrame", "args", ToTrace(args));
-  DCHECK(state_machine_.begin_impl_frame_state() ==
-         SchedulerStateMachine::BEGIN_IMPL_FRAME_STATE_IDLE);
+  DCHECK_EQ(state_machine_.begin_impl_frame_state(),
+            SchedulerStateMachine::BEGIN_IMPL_FRAME_STATE_IDLE);
   DCHECK(state_machine_.HasInitializedOutputSurface());
 
   advance_commit_state_task_.Cancel();