Reland "Allow unspecified max allocatable bitrate in VideoSendStreamImpl"

This is a reland of commit e844bd819c4663de5422be9c2cdb25204b03b41b

The changes after Patchset 1 are intended to fix a corner case where a
video send stream config is used where layers (simulcast or svc) are
configured and active but have undefined max bitrate. In those cases the
max bitrate will be inherited from the global max bitrate.

Original change's description:
> Allow unspecified max allocatable bitrate in VideoSendStreamImpl
>
> If we have no video the can be enabled, based on the VideoEncoderConfig
> (e.g. the max bitrate is <= 0 or all simulcast streams are
> `active=false`) - let the max bitrate be reported as 0 for the sake of
> bandwidth allocation (MediaStreamAllocationConfig).
>
> Previous to this CL, the above conditions would result in an arbitrary
> value of 10Mbps to use as the max allocation limit. When creating a peer
> connection with audio and video configured but the video disabled/muted,
> that would result in the bandwidth estimator trying to send packet
> trains to probe up to 10Mbps, potentially impacting the network and
> disturbing audio even though we have no need for such as high BWE.
>
> Bug: webrtc:494350649
> Change-Id: Id6353fd93c6170610a790c084d75f3b3c5c3ee97
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/449100
> Reviewed-by: Per Kjellander <perkj@webrtc.org>
> Commit-Queue: Erik Språng <sprang@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#47220}

Bug: webrtc:494350649
Change-Id: I59e415bbd121b8661dc5b53e67a2788a335539a9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/459680
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47245}