WebGL: use storage texture for DrawingBuffer only if CMAA is enabled.

It's because Linux ATI bot fails
WebglConformance.conformance_textures_misc_tex_image_webgl
When finding the solution for ATI driver, rollback this patch.

BUG=557848
TEST=WebglConformance.conformance_textures_misc_tex_image_webgl
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel;tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2131863003
Cr-Commit-Position: refs/heads/master@{#404514}
1 file changed