This saves us looking up the FormatSet repeatedly.

BUG=angleproject:1455

Change-Id: I77890c1eb427e7d087ceaf194a5001b8b03585d4
Reviewed-on: https://chromium-review.googlesource.com/359084Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>