Implement proper synchronization between HW video decode IPC and CommandBuffer.

content/common/gpu/gpu_command_buffer_stub.h

Index: content/common/gpu/gpu_command_buffer_stub.h
diff --git a/content/common/gpu/gpu_command_buffer_stub.h b/content/common/gpu/gpu_command_buffer_stub.h
index fa8d7eeb969cdd37c0a2076e85c3f034b34b9709..5fdb0e930f497beab8c7456e1ded330a5413a459 100644
--- a/content/common/gpu/gpu_command_buffer_stub.h
+++ b/content/common/gpu/gpu_command_buffer_stub.h
@@ -63,6 +63,12 @@ class GpuCommandBufferStub
   // to the same renderer process.
   int32 route_id() const { return route_id_; }
 
+  // Return the current token in the underlying command buffer, or 0 if not yet
+  // initialized.
+  int32 token() const {
+    return command_buffer_.get() ? command_buffer_->GetState().token : 0;
+  }
+
 #if defined(OS_WIN)
   // Called only by the compositor window's window proc
   void OnCompositorWindowPainted();
@@ -84,6 +90,11 @@ class GpuCommandBufferStub
   // unblock itself and handle pending messages.
   void CommandBufferWasDestroyed();
 
+  // Register a callback to be Run() whenever the underlying scheduler receives
+  // a set_token() call.  The callback will be Run() with the just-set token as
+  // its only parameter.  Multiple callbacks may be registered.
+  void AddSetTokenCallback(const base::Callback<void(int32)>& callback);
+
  private:
   // Message handlers:
   void OnInitialize(base::SharedMemoryHandle ring_buffer,
@@ -126,6 +137,9 @@ class GpuCommandBufferStub
   void ResizeCallback(gfx::Size size);
   void ReportState();
 
+  // Callback registered with GpuScheduler to receive set_token() notifications.
+  void OnSetToken(int32 token);
+
   // The lifetime of objects of this class is managed by a GpuChannel. The
   // GpuChannels destroy all the GpuCommandBufferStubs that they own when they
   // are destroyed. So a raw pointer is safe.
@@ -147,6 +161,7 @@ class GpuCommandBufferStub
   scoped_ptr<gpu::CommandBufferService> command_buffer_;
   scoped_ptr<gpu::GpuScheduler> scheduler_;
   std::queue<IPC::Message*> deferred_messages_;
+  std::vector<base::Callback<void(int32)> > set_token_callbacks_;
 
   // SetParent may be called before Initialize, in which case we need to keep
   // around the parent stub, so that Initialize can set the parent correctly.