We had accidentally blocked mipmap generation for all unsized
formats in ES3, because they were marked as non-renderable.
The spec specifically allows for mipmaps with unsized formats
in GenerateMipmap.

BUG=angle:550
Change-Id: Iee7eddd011f4fe86b690422723620593f8049ba6
Reviewed-on: https://chromium-review.googlesource.com/186971Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>