This saves us re-creating vector memory every iteration, and allows
us to iterate over much smaller lists of attributes. In the future
it could allow us to update the cache more efficiently with state
change updates.

BUG=angleproject:959

Change-Id: Ie8ae7a31726468dc2184165380f1f3e5e0152936
Reviewed-on: https://chromium-review.googlesource.com/277284Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>