[Cast Streaming] Add about:flag for metrics overlay.

The Cast Streaming sender can render a metrics overlay on outgoing video
frames with many useful performance metrics. This used to be enabled
through VLOG settings, which are hard for end users to change and also
hard to change on ChromeOS.

Instead just use a feature flag for this overlay.

Bug: 1025232
Change-Id: Ia91b7fcdb4988a474064445766a08d6480c9527e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4601985
Reviewed-by: Jordan Bayles <jophba@chromium.org>
Commit-Queue: Mark Foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1155637}