When using glCopyTexImage2D clearing of the mip level needs to happen
when running in WebGL or robust resource init mode and any pixel would
be sampled outside of the framebuffer. Previously the code was using
"setImage" for this purpose, causing issues when a PIXEL_UNPACK_BUFFER
was bound.

Also add a regression test.

BUG=chromium:827667

Change-Id: I03be20d8272730ab30afdab2f8919be853e729b6
Reviewed-on: https://chromium-review.googlesource.com/1000182
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>