Add a zero-filled scratch buffer to Context. We need this in a few places for handling resource init. Centralize this in the context so we don't have to recreate and re-fill a large zero buffer. BUG=angleproject:2107 Change-Id: Icf9ce417e7ee3498f03e47741dfff89e81b49519 Reviewed-on: https://chromium-review.googlesource.com/576057 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>