Add defaults to OpenScreen enum conversions

This CL adds a default label to OpenScreen <=> Media enum conversions,
to avoid breaking builds when Chromium or Open Screen adds new enums.

This CL also adds kSampleFormatPlanarU8 support to SampleFormat.

Fixed: 1231734
Change-Id: I2cf9860ddc35b72ba8e18536d91ff3dac50672d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3069395
Auto-Submit: Thomas Guilbert <tguilbert@chromium.org>
Commit-Queue: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Reviewed-by: Jordan Bayles <jophba@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908602}