Ensure the underlying GL does not modify areas of the texture that
correspond to areas outside the framebuffer, as required for WebGL.
Also zero out the texture in WebGL mode because CopyTexImage must return
zeroes for areas outside the framebuffer.

Enable corresponding test.

BUG=angleproject:1815

Change-Id: I51b1221dbf0dda0952e2ae89ee6ac925b5d1d4a4
Reviewed-on: https://chromium-review.googlesource.com/551535
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>