Add note about frame changes to NavigationHandle::GetRenderFrameHost()

This change adds an additional note to the comment for this method
highlighting the fact that the RenderFrameHost may change during
navigation and that this method only returns the final host. To observe
the change WebContentsObserver::RenderFrameHostChanged() should be used.

Bug: 898645
Change-Id: I716923758fae621c596b599c754e998d86324d20
Reviewed-on: https://chromium-review.googlesource.com/c/1330757
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608633}

content/public/browser/navigation_handle.h

@@ -173,9 +173,11 @@ class CONTENT_EXPORT NavigationHandle {
   virtual net::Error GetNetErrorCode() = 0;
 
   // Returns the RenderFrameHost this navigation is committing in.  The
-  // RenderFrameHost returned will be the final host for the navigation.  This
-  // can only be accessed after a response has been delivered for processing,
-  // or after the navigation fails with an error page.
+  // RenderFrameHost returned will be the final host for the navigation. (Use
+  // WebContentsObserver::RenderFrameHostChanged() to observe RenderFrameHost
+  // changes that occur during navigation.) This can only be accessed after a
+  // response has been delivered for processing, or after the navigation fails
+  // with an error page.
   virtual RenderFrameHost* GetRenderFrameHost() = 0;
 
   // Whether the navigation happened without changing document. Examples of