Mark RenderFrameImple::routing_id_ as const.

Change-Id: I988b82b5393cb4e2a6df2439e52cd864be31e930
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2139853
Auto-Submit: Fergal Daly <fergal@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757168}

content/renderer/render_frame_impl.h

@@ -1365,7 +1365,7 @@ class CONTENT_EXPORT RenderFrameImpl
   bool in_frame_tree_;
 
   RenderViewImpl* render_view_;
-  int routing_id_;
+  const int routing_id_;
 
   // If this RenderFrame was created to replace a previous object, this will
   // store its routing id. The previous object can be: