This should make insertion much faster, which will improve the speed
of many BindBuffer calls.

BUG=angleproject:1458

Change-Id: I3f7ebc02cc481257be8b84773506b9ac9e3be677
Reviewed-on: https://chromium-review.googlesource.com/364221Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>