We would advertise ES3 in some cases when we didn't support it.

BUG=angleproject:1381
BUG=chromium:649101

Change-Id: I2a5bbc1d6153cdfc6c9c577aa0b82caf8482a42d
Reviewed-on: https://chromium-review.googlesource.com/388190
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>