Use a pseudo-extension CHROMIUM_framebuffer_mixed_samples

Use the pseudo-extension CHROMIUM_framebuffer_mixed_samples when
run with Chromium command buffer. The extension exposes
NV_framebuffer_mixed_samples subset that Skia needs in order to
use NV_path_rendering with mixed samples.

BUG=506765

Review URL: https://codereview.chromium.org/1507373004
3 files changed