CopyTextureChromium sometimes failed because of incorrect image dimensions
being used in the image copy command.  Simply use 0, the images are all
tightly packed.

BUG=angleproject:2723

Change-Id: I2332561bdc9ee70ba07101bcf2db7ea9f2355844
Reviewed-on: https://chromium-review.googlesource.com/1194907
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>