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

content/renderer/render_widget.h

Index: content/renderer/render_widget.h
diff --git a/content/renderer/render_widget.h b/content/renderer/render_widget.h
index d9c9535f08771d11b33aa3908e263b31774e0a71..01b4fdba58c6380899b032492765062d6bb7c550 100644
--- a/content/renderer/render_widget.h
+++ b/content/renderer/render_widget.h
@@ -91,6 +91,10 @@ class CONTENT_EXPORT RenderWidget
     return routing_id_;
   }
 
+  int32 surface_id() const {
+    return surface_id_;
+  }
+
   // May return NULL when the window is closing.
   WebKit::WebWidget* webwidget() const { return webwidget_; }
 
@@ -338,6 +342,8 @@ class CONTENT_EXPORT RenderWidget
   // RenderWidgetHost. When MSG_ROUTING_NONE, no messages may be sent.
   int32 routing_id_;
 
+  int32 surface_id_;
+
   // We are responsible for destroying this object via its Close method.
   WebKit::WebWidget* webwidget_;