This will allow us to more easily take advantage of Chromium's
MRUCache class.

BUG=angleproject:2044

Change-Id: I3fad82fc825861dc1c2095f25da83159da76e76b
Reviewed-on: https://chromium-review.googlesource.com/517359Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>