Remove GSC usage from ExynosVideoDecodeAccelerator.

With support for compositing from GL_TEXTURE_EXTERNAL_OES image (and support
from the Exynos GL stack for creating those textures), the video decoder
does not have to perform an explicit YUV->RGB color conversion step.

+27% throughput on birds0.h264 through vda_unittest.

BUG=167417
TEST=local build, run on CrOS snow, vda_unittest
TEST=local build, run on Android/clank

Review URL: https://codereview.chromium.org/23526070

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235128 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235128