[MSC] Add policy to allow auto-select all screens for gDisplayMediaSet.

Add a policy that can be used to enable auto-select of all screens for
defined origins with the getDisplayMediaSet API. The design doc can be
found under go/multiscreencapture.

Bug: 1300883
Change-Id: I9cc10881c4c8f09d8667868471e681dff1289c57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3481096
Reviewed-by: Elad Alon <eladalon@chromium.org>
Reviewed-by: Kamila Hasanbega <hkamila@chromium.org>
Reviewed-by: Alexander Hendrich <hendrich@chromium.org>
Reviewed-by: Theodore Olsauskas-Warren <sauski@google.com>
Reviewed-by: Cait Phillips <caitkp@chromium.org>
Commit-Queue: Simon Hangl <simonha@google.com>
Cr-Commit-Position: refs/heads/main@{#988062}