This saves a lot of extra work when updating a Texture.

Bug: angleproject:2763
Change-Id: I87e310ef4f847713123bd24711e1166949ff95d2
Reviewed-on: https://chromium-review.googlesource.com/c/1254043Reviewed-by: Yuly Novikov <ynovikov@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>