Don't recompute the cached attribute layout for a program if the
vertex array info hasn't changed. We can use the Serial class to
know when a vertex array has identical state.

BUG=angleproject:1156

Change-Id: Ia11f6ac268f63c3299f6d6d80c2866009cb8429c
Reviewed-on: https://chromium-review.googlesource.com/529768Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>