We can use pointer math here, instead of copying values.

BUG=angleproject:959

Change-Id: I3b87956224d0846c9011f5d8edb811bc5e4f2b85
Reviewed-on: https://chromium-review.googlesource.com/277119Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>