Fix for disabled color writes xp on n6

The N6 has some driver bug that is required us to give some value to gl_FragColor.
The disabledColorWriteXP should not need to set any value in its shader, but to fix
this crash this cl sets gl_FragColor to 0.

BUG=445377

Review URL: https://codereview.chromium.org/823053003
1 file changed