Notify the embedder when FrameLoader::stopAllLoaders is called.

The motivation is that out of process plugins get notified when the user presses the Stop button in the browser or when window.stop() is called, so that the plugin process can cancel its requests as well.

BUG=286074

Review URL: https://chromiumcodereview.appspot.com/24243012

git-svn-id: svn://svn.chromium.org/blink/trunk@158200 bbb929c8-8fbe-4397-9dbb-9b2b20218538