gpu: reference target surfaces through a globally unique surface id.

content/browser/renderer_host/render_widget_host.h

Index: content/browser/renderer_host/render_widget_host.h
diff --git a/content/browser/renderer_host/render_widget_host.h b/content/browser/renderer_host/render_widget_host.h
index 422bedaf666705f2bc36000b374ae57e6a2ea048..d65a903876dd95d43e5ad172c68e808bfb0bc3ab 100644
--- a/content/browser/renderer_host/render_widget_host.h
+++ b/content/browser/renderer_host/render_widget_host.h
@@ -160,6 +160,7 @@ class CONTENT_EXPORT RenderWidgetHost : public IPC::Channel::Listener,
 
   content::RenderProcessHost* process() const { return process_; }
   int routing_id() const { return routing_id_; }
+  int surface_id() const { return surface_id_; }
   bool renderer_accessible() { return renderer_accessible_; }
 
   bool empty() const { return current_size_.IsEmpty(); }
@@ -654,6 +655,9 @@ class CONTENT_EXPORT RenderWidgetHost : public IPC::Channel::Listener,
   // The ID of the corresponding object in the Renderer Instance.
   int routing_id_;
 
+  // The ID of the surface corresponding to this render widget.
+  int surface_id_;
+
   // Indicates whether a page is loading or not.
   bool is_loading_;