Reland "Add about flag for enabling the PipeWire support in WebRTC"

This is a reland of 8788015d2b2efa354c641f4e0ed38fba726f3ae7

Unit tests are fixed in the reland.

Original change's description:
> Add about flag for enabling the PipeWire support in WebRTC
>
> Add a new enable-webrtc-pipewire-capturer about flag, that when
> enabled, will allow to use the PipeWire media server for providing
> the screen content for screen casting when user is running on Wayland
> display server on Linux.
>
> The flag is disabled by default, due to the user experience as it
> involves two extra confirmation dialogs outside of Chromium - requested
> by Weiyong Yao. It could change the default, once it goes through Chromium
> UI review.
>
> Bug: 682122
> Reviewed-on: https://chromium-review.googlesource.com/c/1365444
> Reviewed-by: Weiyong Yao <[email protected]>
> Reviewed-by: Jochen Eisinger <[email protected]>
> Commit-Queue: Tomáš Popela <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#615886}

Bug: 682122
Change-Id: Ie5d0bf4945f621121c14e95a4addad3afac5bec1
Reviewed-on: https://chromium-review.googlesource.com/c/1402441
Reviewed-by: Weiyong Yao <[email protected]>
Reviewed-by: Jochen Eisinger <[email protected]>
Reviewed-by: Avi Drissman <[email protected]>
Commit-Queue: Tomáš Popela <[email protected]>
Cr-Commit-Position: refs/heads/master@{#622491}
11 files changed