Enable NV12 VideoFrames on Mac.

As described in crbug.com/524582, we observed a reduction in power
consumption when this format is used to display videos directly using
the WindowServer.

The downside of this format is that a conversion from YUV to RGB will
be performed on the GPU using GL when the VideoFrame has to be used
from GL.

BUG=524582

Review URL: https://codereview.chromium.org/1686443002

Cr-Commit-Position: refs/heads/master@{#374825}