Move disk cache entry logic to its own entity.

Add CacheStorageCacheEntryHandler which handles how blobs are written
to the disk cache and how they are read back.

This CL causes no behavioral changes, it is just a refactor.

CacheStorageCacheEntryHandler can now be specialized for Background
Fetch caches to store request body blobs instead of side blobs.

Bug: 774054
Change-Id: I4bea29ea53ea898e2b3e517fc3b0a74777280d16
Reviewed-on: https://chromium-review.googlesource.com/c/1338188
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Ben Kelly <wanderview@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610084}
5 files changed