AppCache: Extract host methods from mojom::AppCacheBackend.

The AppCacheBackend interface had a lot of methods that took a host_id.
This CL moves these methods into an AppCacheHost interface.

Bug: 611938
Change-Id: Iaff67662efce0d588bec31cdecb192cf5294c26e
Reviewed-on: https://chromium-review.googlesource.com/c/1462659
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Staphany Park <staphany@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631886}
13 files changed