Detect <link rel='manifest'> and notify embedder.

WebFrameClient gets a dispatchManifestChange method that will
be called by the FrameLoaderClient when the manifest
information might have changed. The embedder is not aware of
the manifest URL.

BUG=366145

Review URL: https://codereview.chromium.org/249633002

git-svn-id: svn://svn.chromium.org/blink/trunk@173664 bbb929c8-8fbe-4397-9dbb-9b2b20218538