[AppCache] Make AppCacheFrontend be a per-frame mojo interface.

This should make onion souping slightly easier, but also gets us closer
to making sure that tasks run on the correct frame associated task
runners. In a follow-up CL the AppCacheFrontend mojo binding can be
changed to use correct per frame task runners.

Also removes AppCacheFrontendProxy (in the browser) and
AppCacheFrontendImpl (in the renderer), as those layers of indirection
are no longer needed.

Bug: 611938
Change-Id: I4d50a0f13f0d2624071b3c1734457be02483df60
Reviewed-on: https://chromium-review.googlesource.com/c/1478009
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634295}
31 files changed