Add a NavigationThrottle to the public content/ interface

content/browser/web_contents/web_contents_impl.cc

Index: content/browser/web_contents/web_contents_impl.cc
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index a93eea7e70809ddf518856e59d4eeb4eb79a8e96..b1e1b23e429d6827fb3f82df335c8c47a6b49016 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -2778,6 +2778,7 @@ void WebContentsImpl::SetFocusToLocationBar(bool select_all) {
 }
 
 void WebContentsImpl::DidStartNavigation(NavigationHandle* navigation_handle) {
+  static_cast<NavigationHandleImpl*>(navigation_handle)->set_web_contents(this);

[nasko, 2015/09/14 22:00:04]

Is this the only usage of set_web_contents? Can we avoid having this settable
through a public method if noone else needs it?

[clamy, 2015/09/14 22:12:10]

Well if I have a handy way to get to the WebContents from somewhere inside
/frame_host yes, otherwise that requires making WebContentsImpl a friend of
NavigationHandleImpl.

[Charlie Reis, 2015/09/16 00:09:24]

Yeah, this is one of the tricky aspects of having frame_host code know about
WebContents.  I see that it's needed, though, since NavigationHandle really is
tied to WebContentsObserver.  Not sure if I see a better way to handle it than
this.

   FOR_EACH_OBSERVER(WebContentsObserver, observers_,
                     DidStartNavigation(navigation_handle));
 }