When switching VAOs, if we switch to a VAO which has disabled
attributes, we could occasionally in some edge cases not have a buffer
initialized to render with. Fix this by re-checking the current
value (disabled) attributes every VAO switch.

Probably a regression caused by d28758de:
"D3D11: Re-enable updateVertexBuffer dirty bits."

BUG=chromium:778689

Change-Id: I01814bafa1d6e3a7d6a5c03bc5d058f39346f087
Reviewed-on: https://chromium-review.googlesource.com/748426Reviewed-by: Jamie Madill <jmadill@chromium.org>