Fixes a bug where subsequent updates to a "current value" (disabled)
Vertex Attribute would not trigger the state change to D3D11 such that
the updated buffer handle would be applied to D3D11. Also adds a test
to cover the problem case.

This bug was introduced in 2bc94733:
"D3D11: Minor optimizations to vertex attribute application."

BUG=chromium:779675
BUG=angleproject:1155

Change-Id: Ib0447231fb6969e7dad1e1e576315cab91acf4f4
Reviewed-on: https://chromium-review.googlesource.com/744924
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>