[Cast Streaming] Add *very* experiment HEVC support

This patch begins the (probably) long road to enabling HEVC support
in Cast Streaming, by adding a feature flag and allowing it to be manually enabled in chrome://flags.

\#cast-streaming-hardware-hevc

Change-Id: I89373a67984fb3b2c8240c5f02f5be3202f4f9aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6442545
Reviewed-by: Frank Liberato <liberato@chromium.org>
Commit-Queue: Jordan Bayles <jophba@chromium.org>
Reviewed-by: Muyao Xu <muyaoxu@google.com>
Cr-Commit-Position: refs/heads/main@{#1454441}