Add some const to content/.

content/browser/frame_host/render_frame_host_impl.h

Index: content/browser/frame_host/render_frame_host_impl.h
diff --git a/content/browser/frame_host/render_frame_host_impl.h b/content/browser/frame_host/render_frame_host_impl.h
index 74ee70b1b2541002d192b8df3fbf34ed5a768c1a..70173f9f7587480c6d6f79a7e96c12704625a4c8 100644
--- a/content/browser/frame_host/render_frame_host_impl.h
+++ b/content/browser/frame_host/render_frame_host_impl.h
@@ -885,28 +885,28 @@ class CONTENT_EXPORT RenderFrameHostImpl
   // we have a RenderViewHost for each RenderFrameHost.
   // TODO(creis): RenderViewHost will eventually go away and be replaced with
   // some form of page context.
-  RenderViewHostImpl* render_view_host_;
+  RenderViewHostImpl* const render_view_host_;
 
-  RenderFrameHostDelegate* delegate_;
+  RenderFrameHostDelegate* const delegate_;
 
   // The SiteInstance associated with this RenderFrameHost. All content drawn
   // in this RenderFrameHost is part of this SiteInstance. Cannot change over
   // time.
-  scoped_refptr<SiteInstanceImpl> site_instance_;
+  const scoped_refptr<SiteInstanceImpl> site_instance_;
 
   // The renderer process this RenderFrameHost is associated with. It is
   // equivalent to the result of site_instance_->GetProcess(), but that
   // method has the side effect of creating the process if it doesn't exist.
   // Cache a pointer to avoid unnecessary process creation.

[clamy, 2017/03/06 14:38:44]

@nasko: the comment here seems weird. It says that we want to avoid calling
site_instance_->GetProcess(), but we initialize this member precisely by calling
this function. Does it still make sense to keep this member around?

[nasko, 2017/03/06 19:46:15]

I think the goal here is not to avoid it during initialization. It is perfectly
possible for the renderer process to crash and be gone. Any code trying to use
the process_ after this point should not accidentally recreate it, as it might
not actually be used. 
If you think this is useful piece of data to put in the comment, I'm more than
happy to approve a CL : ).

[clamy, 2017/03/09 13:44:34]

I see. We should probably update the comment then.

-  RenderProcessHost* process_;
+  RenderProcessHost* const process_;
 
   // Reference to the whole frame tree that this RenderFrameHost belongs to.
   // Allows this RenderFrameHost to add and remove nodes in response to
   // messages from the renderer requesting DOM manipulation.
-  FrameTree* frame_tree_;
+  FrameTree* const frame_tree_;
 
   // The FrameTreeNode which this RenderFrameHostImpl is hosted in.
-  FrameTreeNode* frame_tree_node_;
+  FrameTreeNode* const frame_tree_node_;
 
   // The active parent RenderFrameHost for this frame, if it is a subframe.
   // Null for the main frame.  This is cached because the parent FrameTreeNode