Add a WebRemoteFrameClient interface to public/web

This interface temporarily subclasses WebFrameClient to ease the transition, and will be implemented by content::RenderFrameProxy. Add postMessageEvent(), to be properly plumbed in a followup patch.

This is step 1 of 3 on crubg.com/411066

BUG=411066

Review URL: https://codereview.chromium.org/539413002

git-svn-id: svn://svn.chromium.org/blink/trunk@181569 bbb929c8-8fbe-4397-9dbb-9b2b20218538