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

gpu/command_buffer/service/gpu_scheduler.h

Index: gpu/command_buffer/service/gpu_scheduler.h
diff --git a/gpu/command_buffer/service/gpu_scheduler.h b/gpu/command_buffer/service/gpu_scheduler.h
index 32a38b9de0873516a74d735527bc6856778b4fee..5a38c77391d184a960b7777a6d208a0c3f64f225 100644
--- a/gpu/command_buffer/service/gpu_scheduler.h
+++ b/gpu/command_buffer/service/gpu_scheduler.h
@@ -80,6 +80,10 @@ class GPU_EXPORT GpuScheduler
   // Takes ownership of callback object.
   void SetScheduledCallback(const base::Closure& scheduled_callback);
 
+  // Sets a callback that is invoked just before scheduler is descheduled.
+  // Takes ownership of callback object.
+  void SetDescheduledCallback(const base::Closure& descheduled_callback);
+
   // Implementation of CommandBufferEngine.
   virtual Buffer GetSharedMemoryBuffer(int32 shm_id) OVERRIDE;
   virtual void set_token(int32 token) OVERRIDE;
@@ -147,6 +151,7 @@ class GPU_EXPORT GpuScheduler
   std::queue<linked_ptr<UnscheduleFence> > unschedule_fences_;
 
   base::Closure scheduled_callback_;
+  base::Closure descheduled_callback_;
   base::Closure command_processed_callback_;
 
   // If non-NULL and |preemption_flag_->IsSet()|, exit PutChanged early.