Update from https://crrev.com/309717

cc/scheduler/scheduler.cc

 // Copyright 2011 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 "cc/scheduler/scheduler.h"
 
 #include <algorithm>
 
 #include "base/auto_reset.h"
 #include "base/debug/trace_event.h"
@@ skipping 438 matching lines @@
   state_machine_.SetChildrenNeedBeginFrames(children_need_begin_frames);
   ProcessScheduledActions();
 }
 
 // BeginRetroFrame is called for BeginFrames that we've deferred because
 // the scheduler was in the middle of processing a previous BeginFrame.
 void Scheduler::BeginRetroFrame() {
   TRACE_EVENT0("cc", "Scheduler::BeginRetroFrame");
   DCHECK(!settings_.using_synchronous_renderer_compositor);
   DCHECK(begin_retro_frame_posted_);
+  DCHECK_EQ(state_machine_.begin_impl_frame_state(),
+            SchedulerStateMachine::BEGIN_IMPL_FRAME_STATE_IDLE);
   begin_retro_frame_posted_ = false;
 
   // If there aren't any retroactive BeginFrames, then we've lost the
   // OutputSurface and should abort.
   if (begin_retro_frame_args_.empty())
     return;
 
   // Discard expired BeginRetroFrames
   // Today, we should always end up with at most one un-expired BeginRetroFrame
   // because deadlines will not be greater than the next frame time. We don't
@@ skipping 332 matching lines @@
 }
 
 bool Scheduler::IsBeginMainFrameSentOrStarted() const {
   return (state_machine_.commit_state() ==
               SchedulerStateMachine::COMMIT_STATE_BEGIN_MAIN_FRAME_SENT ||
           state_machine_.commit_state() ==
               SchedulerStateMachine::COMMIT_STATE_BEGIN_MAIN_FRAME_STARTED);
 }
 
 }  // namespace cc