[BreakoutBox] Update queue management in MediaStreamTrackProcessor

This CL updates queue management for MSTP as follows:

1. The main queue management now occurs in the underlying source and
not in the stream controller. A frame is enqueued in the controller
when pull() is invoked, or a when a new frame becomes available after
a previous pull with an empty queue.

2. When the queue is full and more frames arrive (i.e., in a situation
of slow consumption), old frames are dropped. Before this CL, incoming
frames were dropped when the controller queue desired size exceeded a
threshold.

3. The queue size is now configurable from the JS side (via a
constructor parameter). This makes it possible to support more use
cases than with a fixed queue. The new parameter will also be useful
for audio when it becomes available.

Fixed: 1157605, 1157604
Change-Id: I0260f7d5fe0495c33cfc022ce534ef04c518e90b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587034
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836229}