This allows us to simplify the logic in VertexDataManager enough
that we can start to cache attribute information.

BUG=angleproject:959

Change-Id: I7b53a137d73f40f86e3acb9caebb66f9cacf8b6f
Reviewed-on: https://chromium-review.googlesource.com/277283Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>