WebGL: Forbid GenerateMipmap on zero-size textures

This is a WebGL spec change:
https://github.com/KhronosGroup/WebGL/commit/bfbe124a7bb92eed34ea7cc18694167ed66b1030

Fixes conformance2/textures/misc/tex-mipmap-levels.html

Bug: 898351
Change-Id: I710edd7f189d21cf080d5b775a2c8aef30ad166c
Reviewed-on: https://chromium-review.googlesource.com/c/1352494
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611586}
2 files changed