During draw calls, we wouldn't add the current attrib offset to the
required draw call size when checking attributes. This could lead to
us producing warnings in the D3D11 runtime, and miss returning some
errors.

BUG=angleproject:1339

Change-Id: I03555be396df46f83d96dfb34fbcb145169625e8
Reviewed-on: https://chromium-review.googlesource.com/331807Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>