Interface for associating in-memory hint bytes with disk cache entries.
Implementation for it in the mock cache (with off switch)

The intent is to implement it in simple cache, and to use it in
HttpCache::Transaction to avoid opening entries that would not be usable
under present load flags due to caching-hostile headers.

BUG=729679

Review-Url: https://codereview.chromium.org/2958033002
Cr-Commit-Position: refs/heads/master@{#498491}
4 files changed