We would allow direct storage when this format is emulated. Noticed
this when making a test which used a single float vertex format.

BUG=angleproject:1635

Change-Id: Ia2164b0f311b27f24b6aa7760eb9f9a3834c6044
Reviewed-on: https://chromium-review.googlesource.com/451578Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>