Prepares for caching hasMappedBuffer. Also inclines several checks for
faster speed.

Bug: angleproject:2746
Change-Id: I74f9408d7b41e245c3f58d367dd2cc8fbace4a7a
Reviewed-on: https://chromium-review.googlesource.com/1150762
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>