Avoid calling a getter repeatedly in a loop, when we can instead
store a local reference to an array.

BUG=angleproject:959

Change-Id: I507ad1a6cf3bb6183dd3499df024dfec6950a6c8
Reviewed-on: https://chromium-review.googlesource.com/277285Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>