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

content/common/gpu/gpu_channel.h

Index: content/common/gpu/gpu_channel.h
diff --git a/content/common/gpu/gpu_channel.h b/content/common/gpu/gpu_channel.h
index 3cf2cafe4f51a06176c81ace1e17503cfbc32030..9574f10aae0743205d84feea0accd53869783deb 100644
--- a/content/common/gpu/gpu_channel.h
+++ b/content/common/gpu/gpu_channel.h
@@ -102,6 +102,11 @@ class GpuChannel : public IPC::Listener,
   // 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,
@@ -240,6 +245,8 @@ class GpuChannel : public IPC::Listener,
   scoped_refptr<SyncPointMessageFilter> filter_;
   scoped_refptr<base::MessageLoopProxy> io_message_loop_;
 
+  bool a_stub_is_descheduled_;
+
   DISALLOW_COPY_AND_ASSIGN(GpuChannel);
 };