This bug showed up on Mac which doesn't support the extension.  The internalFormat
is correctly translated to the workaround format but the format and type are not.

BUG=angleproject:1958

Change-Id: I9b31f0491df1bb623bf59996cbb02d0276b6355c
Reviewed-on: https://chromium-review.googlesource.com/488582Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>