Using the ELEMENT_ARRAY binding can cause issues such as rebinding the
current VAO's index buffer unexpectedly.

BUG=angleproject:881

Change-Id: Ieeed52ac81c23a46bfe9fc4365239c2b177ff6ac
Reviewed-on: https://chromium-review.googlesource.com/261563Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>