Allow unsized RGB/UNSIGNED_BYTE without the rgb8rgba8 extension.

BUG=angleproject:1523

Change-Id: Ic7b4236c7b8389952b093719f26840ea2bbd01bf
Reviewed-on: https://chromium-review.googlesource.com/742501
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>