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
Change-Id: I70c07a4a170b2bf1717f6c155905625eba84ff5a
Reviewed-on: https://chromium-review.googlesource.com/c/1365444
Reviewed-by: Weiyong Yao <braveyao@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Tomáš Popela <tomas.popela@gmail.com>
Cr-Commit-Position: refs/heads/master@{#615886}
9 files changed