Check client_ isn't null in GpuCompositorframeSink.

services/ui/surfaces/gpu_compositor_frame_sink.cc

 // Copyright 2015 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 "services/ui/surfaces/gpu_compositor_frame_sink.h"
 
 #include "services/ui/surfaces/display_compositor.h"
 
 namespace ui {
 
@@ skipping 30 matching lines @@
   support_.SetNeedsBeginFrame(needs_begin_frame);
 }
 
 void GpuCompositorFrameSink::SubmitCompositorFrame(
     const cc::LocalFrameId& local_frame_id,
     cc::CompositorFrame frame) {
   support_.SubmitCompositorFrame(local_frame_id, std::move(frame));
 }
 
 void GpuCompositorFrameSink::DidReceiveCompositorFrameAck() {
-  client_->DidReceiveCompositorFrameAck();
+  if (client_)
+    client_->DidReceiveCompositorFrameAck();
 }
 
 void GpuCompositorFrameSink::AddChildFrameSink(
     const cc::FrameSinkId& child_frame_sink_id) {
   support_.AddChildFrameSink(child_frame_sink_id);
 }
 
 void GpuCompositorFrameSink::RemoveChildFrameSink(
     const cc::FrameSinkId& child_frame_sink_id) {
   support_.RemoveChildFrameSink(child_frame_sink_id);
@@ skipping 23 matching lines @@
 }
 
 void GpuCompositorFrameSink::OnPrivateConnectionLost() {
   private_connection_lost_ = true;
   // Request destruction of |this| only if both connections are lost.
   display_compositor_->OnCompositorFrameSinkPrivateConnectionLost(
       support_.frame_sink_id(), client_connection_lost_);
 }
 
 }  // namespace ui