Add variables to lend unused audio bits to video

This CL only adds variables necessary for the feature, which will be
implemented in later CLs.

Bug: webrtc:350555527
Change-Id: I71e56666e629f56168d316bf693150c0df0e2ecf
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/356740
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Dan Tan <dwtan@google.com>
Reviewed-by: Jakob Ivarsson‎ <jakobi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42698}