Add a back pressure signal for media remoting

[takumif]

Per the TPAC discussion, when media remoting for Remote Playback, there should be a back-pressure signal to indicate whether or not the media bitrate is too high given the network/receiver constraints (e.g. bandwidth, buffer size).

w3c/remote-playback#132 tracks this on the Remote Playback API side.

[markafoltz]

From https://www.w3.org/2019/09/16-webscreens-minutes.html#x02:

RESOLVED: Add a backpressure signal to the OSP

Resolving this, should close w3c/remote-playback#132.

[anssiko]

ACTION: People to review the PR #248 and provide feedback

via https://www.w3.org/2020/10/20-webscreens-minutes.html#a02

[markafoltz]

See the conversation thread on PR #248 for more background/discussion of this issue.

[markafoltz]

PR #248 was merged back on March 16, and with no additional feedback/concerns, this should be okay to close.