Use kRenderClientId command line flag when starting a render process

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 5bde3b184419e453b372dd7994374c0d46c9717c..6de396e27a2fc411176c59e66fcd46fa1465bcb2 100644
--- a/content/renderer/render_widget.h
+++ b/content/renderer/render_widget.h
@@ -153,6 +153,8 @@ class CONTENT_EXPORT RenderWidget
   // https://crbug.com/545684
   virtual void CloseForFrame();
 
+  int32_t client_id() const { return client_id_; }
+
   int32_t routing_id() const { return routing_id_; }
   void SetRoutingID(int32_t routing_id);
 
@@ -600,6 +602,10 @@ class CONTENT_EXPORT RenderWidget
   // Sends an ACK to the browser process during the next compositor frame.
   void OnWaitNextFrameForTests(int routing_id);
 
+  // Process ID of the browser process. It is used for constructing FrameSinkIds
+  // for the RenderWidgetCompositor
+  int32_t client_id_;
+
   // Routing ID that allows us to communicate to the parent browser process
   // RenderWidgetHost. When MSG_ROUTING_NONE, no messages may be sent.
   int32_t routing_id_;