This moves out some shared logic into a more accessible place. We don't
need to validate buffer overflows when using robust resource access but
we do need to validate we are using client side data correctly.

Bug: angleproject:1391
Change-Id: I7a3dca8409c5a1faf1ff7bc732d5ed1bd62eb3b1
Reviewed-on: https://chromium-review.googlesource.com/1148817
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>