Clean up ApplicationCacheHost after move to public/platform

This patch smushes ApplicationCacheHostInternal into ApplicationCacheHost,
which now implements blink::WebApplicationCacheHostClient.

It also gets rid of things that Chromium no longer depends on (such
as the WebFrame-dependent method of WebApplicationCacheHost).

Depends on Chromium-side patch https://codereview.chromium.org/171773005/

BUG=343272

Review URL: https://codereview.chromium.org/172853003

git-svn-id: svn://svn.chromium.org/blink/trunk@167553 bbb929c8-8fbe-4397-9dbb-9b2b20218538