Make H265 support in WebRTC enabled-by-default.

Approved to ship - all bits are green on the chromestatus entry page:
https://chromestatus.com/feature/5153479456456704

Because H265 is only available when available in HW, and different bots
have different HW support, we're separating the CL for "enable by
default" and CLs for requiring the H265 web platform test to pass with
exceptions depending on HW capabilities as we expect the test CLs to
risk getting temporarily reverted until we get it right (CQ bots and
waterfall bots not being identical have led to surprises in the past).

Bug: chromium:391903235
Change-Id: Ia3041f03aed3481145b3da03d6971c2fe0b52ed9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6367096
Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Mirko Bonadei <mbonadei@chromium.org>
Reviewed-by: Evan Shrubsole <eshr@google.com>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1434157}