We should give the app layer more control over the cache before we
enable it, to save on memory usage in Chrome.

BUG=angleproject:1897

Change-Id: I532c05c3042cb0a2d9c62f362f25d6064042ca2c
Reviewed-on: https://chromium-review.googlesource.com/558370
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>