GPU: Only allow the UI channel to preempt if all stubs are scheduled.

content/common/gpu/gpu_channel.h

 // Copyright (c) 2012 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.
 
 #ifndef CONTENT_COMMON_GPU_GPU_CHANNEL_H_
 #define CONTENT_COMMON_GPU_GPU_CHANNEL_H_
 
 #include <deque>
 #include <string>
 
@@ skipping 84 matching lines @@
   // the queue. Used when the processing of a message gets aborted because of
   // unscheduling conditions.
   void RequeueMessage();
 
   // This is called when a command buffer transitions from the unscheduled
   // state to the scheduled state, which potentially means the channel
   // transitions from the unscheduled to the scheduled state. When this occurs
   // deferred IPC messaged are handled.
   void OnScheduled();
 
+  // This is called when a command buffer transitions between scheduled and
+  // descheduled states. When any stub is descheduled, we stop preempting
+  // other channels.
+  void StubSchedulingChanged();
+
   void CreateViewCommandBuffer(
       const gfx::GLSurfaceHandle& window,
       int32 surface_id,
       const GPUCreateCommandBufferConfig& init_params,
       int32* route_id);
 
   void CreateImage(
       gfx::PluginWindowHandle window,
       int32 image_id,
       gfx::Size* size);
@@ skipping 118 matching lines @@
 
 #if defined(OS_ANDROID)
   scoped_ptr<StreamTextureManagerAndroid> stream_texture_manager_;
 #endif
 
   base::WeakPtrFactory<GpuChannel> weak_factory_;
 
   scoped_refptr<SyncPointMessageFilter> filter_;
   scoped_refptr<base::MessageLoopProxy> io_message_loop_;
 
+  bool a_stub_is_descheduled_;
+
   DISALLOW_COPY_AND_ASSIGN(GpuChannel);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_COMMON_GPU_GPU_CHANNEL_H_