Now that we preserve the sized-ness information, we can validate
CopyTexSubImage and related methods correctly.

Fixed a lot of WebGL 2 tests when using ANGLE.

BUG=angleproject:1228

Change-Id: I959322c0a9bb16a2f16d60dce7cd1e63ca95b45a
Reviewed-on: https://chromium-review.googlesource.com/362618Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>