Implement proper synchronization between HW video decode IPC and CommandBuffer.

gpu/command_buffer/service/gpu_scheduler.cc

Index: gpu/command_buffer/service/gpu_scheduler.cc
diff --git a/gpu/command_buffer/service/gpu_scheduler.cc b/gpu/command_buffer/service/gpu_scheduler.cc
index a67fd3a9a6fae603d5a9de7812cb051b951039ad..1ff60d49b305c4a604656a20aed4d2dc7b350642 100644
--- a/gpu/command_buffer/service/gpu_scheduler.cc
+++ b/gpu/command_buffer/service/gpu_scheduler.cc
@@ -273,6 +273,8 @@ Buffer GpuScheduler::GetSharedMemoryBuffer(int32 shm_id) {
 
 void GpuScheduler::set_token(int32 token) {
   command_buffer_->SetToken(token);
+  if (!set_token_callback_.is_null())
+    set_token_callback_.Run(token);
 }
 
 bool GpuScheduler::SetGetOffset(int32 offset) {
@@ -311,6 +313,12 @@ void GpuScheduler::SetCommandProcessedCallback(
   command_processed_callback_.reset(callback);
 }
 
+void GpuScheduler::SetTokenCallback(
+    const base::Callback<void(int32)>& callback) {
+  DCHECK(set_token_callback_.is_null());
+  set_token_callback_ = callback;
+}
+
 void GpuScheduler::ScheduleProcessCommands() {
   MessageLoop::current()->PostTask(
       FROM_HERE,