According to the OpenGL extension spec EXT_sRGB.txt, EXT_SRGB is based on
ES 2.0 and generateMipmap is not allowed if texture format is SRGB_EXT or
SRGB_ALPHA_EXT.

BUG=769989
TEST=SRGBTextureTest.SRGBValidation*
TEST=SRGBTextureTest.SRGBAValidation*

Change-Id: Ic51da224fcd318187865a44630af6fca5c3ad2de
Reviewed-on: https://chromium-review.googlesource.com/1137924Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Yizhou Jiang <yizhou.jiang@intel.com>