content: Add support for R_8 format to IOSurface backed GpuMemoryBuffers.
This allows native GpuMemoryBuffers on MacOSX to be used
with R_8 format. R_8 format is useful to store YUV planes
of decoded video frames and native GpuMemoryBuffer support
for this format allows for zero-copy transfer of software
decoded video to the graphics hardware.
BUG=485859
TEST=content_unittests --gtest_filter=GpuMemoryBuffer*/1
Review URL: https://codereview.chromium.org/
1132713002
Cr-Commit-Position: refs/heads/master@{#329007}