Ensure the underlying GL does not modify areas of the texture that
correspond to areas outside the framebuffer, as required for WebGL.

Enable corresponding test.

BUG=angleproject:1815

Change-Id: I6092d39e43868902de7ae3aee430deea3b3ff8a1
Reviewed-on: https://chromium-review.googlesource.com/538295
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>