[Mac] Export GetWindowOwnerPid(CGWindowID id).

Export GetWindowOwnerPid to make it accessible for window audio sharing
implementations.

getDisplayMedia() allows users to share a specific window. To capture
audio from that specific window, the audio capturer requires the Process
ID (PID) of the window's owner.

Exporting GetWindowOwnerPid(CGWindowID id) allows the window sharing
logic to find the window owner PID, which is passed to the audio capture
logic to capture audio from the window process.

This has been verified with a POC demonstrating functional window audio
sharing:
https://chromium-review.googlesource.com/c/chromium/src/+/7141659

Bug: chromium:460420529
Change-Id: Idb6ee21dd146a127649e8c23bd1f6b29f0c3b400
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/427301
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Olov Brändström <brandstrom@google.com>
Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46323}