This helper method will be useful for implementing dynamic buffers.

BUG=angle:705

Change-Id: I8461a59724fb8866b36059c9ef5b838f072cc63a
Reviewed-on: https://chromium-review.googlesource.com/213812Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>