Implement MST::BeingTransferred which notifies browser of the transfer

This function calls MSDH::KeepDeviceAliveForTransfer to keep the
MediaStreamDevice alive until it has been cloned successfully in the
transferred context. This function also stops the MST which is being
transferred in the original context.

Bug: 1288839
Change-Id: I29b0a7794409c7f98779c666a1415fd31f9df8c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3812165
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Palak Agarwal <agpalak@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1036032}