Update VertexArrayGL to clear it's cached buffer binding when a buffer is
detached to prevent name aliasing.

Fixes:
 * DEQP standalone crashes.
 * WebGL Conformance DEQP tests.

Change-Id: If21a617629e15873ddeae0368b5399616f2ca68b
Reviewed-on: https://chromium-review.googlesource.com/278050Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>