Make mixed samples contingent on auxiliary extensions

Make mixed samples contingent on auxiliary extensions

Moves the cap for mixed samples into GrShaderCaps and does not enable
it unless we have support for both dual source blending and
multisample disable.

Creates a dedicated cap for multisample disable.

Reconfigures the mixed samples cap to indicate the collective
capability of three different extensions:

  GL_NV_framebuffer_mixed_samples
  GL_NV_sample_mask_override_coverage
  GL_EXT_raster_multisample

Imports tokens and procedures for GL_EXT_raster_multisample.

BUG=skia:

Review URL: https://codereview.chromium.org/1151793002