This can give us much faster draw call validation, by saving us from
checking buffer sizes for non-enabled attribs. Also for checking if
vertex buffers are mapped. Gives >100% increase in the benchmark.

BUG=angleproject:959

Change-Id: I211c310385bdee46ed06f68ecd9c98385e1f8db9
Reviewed-on: https://chromium-review.googlesource.com/267751Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>