Workaround for blacklist KHR_blend_equation_advanced on ARM GPU

ARM driver will check the fragment shader compatiblity for KHR_blend_equation_advanced even if blending is disabled.
Workaround: Set blending equation to any basic equation when we disable blending.
https://code.google.com/p/skia/issues/detail?id=3943

BUG=skia:3943

Review URL: https://codereview.chromium.org/1216963004
2 files changed