Re-use std::hash in RenderStateCache. 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/517359 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>