[M123] Revert "Improve AudioRendererMixer sharing across RenderFrames."

This reverts commit a6068ebab3469d9d7793381987b1168434afaabe.

Reason for revert: Causing too many unforeseen consequences, see
327884890, 327489297

Original change's description:
> Improve AudioRendererMixer sharing across RenderFrames.
>
> It's common for a page to have a lot of YouTube embeds. With the
> current code, none of those embeds share a mixer and thus can
> trigger the 50 physical stream limit quickly.
>
> AFAICT, there's no good reason to prevent sharing of these streams
> across render frames when the default device is used. They are
> output only and an uncompromised renderer can't directly observe
> any effects (readback for WebAudio and MediaStreams happens
> asynchronously below the shared output).
>
> Often the default device is already being shared at the OS level,
> so any demultiplexing we do in the renderer seems pointless.
>
> Looking through the history we've always limited the sharing to
> per RenderFrameID, but it was only out of an abundance of caution
> rather than a particular concern.
>
> For non-default devices there's good reasons we shouldn't share:
> permissions are required per-origin to access the non-default
> device.
>
> R=olka, tguilbert
>
> Bug: b/314766691
> Change-Id: I7ee6865eb675962aff2aeedcd7290666b38c91e5
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5185891
> Commit-Queue: Olga Sharonova <olka@chromium.org>
> Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
> Reviewed-by: Olga Sharonova <olka@chromium.org>
> Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1245750}

(cherry picked from commit 5f20e0e777c3820b92b85fa941a888fe0a50750d)

Bug: b/314766691, b/327489297, b/327884890
Change-Id: Idb20538593c6303900ebc6062501b424974cbb55
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5354149
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1269893}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5356349
Cr-Commit-Position: refs/branch-heads/6312@{#526}
Cr-Branched-From: 6711dcdae48edaf98cbc6964f90fac85b7d9986e-refs/heads/main@{#1262506}

• third_party/blink/renderer/modules/media/audio/audio_renderer_mixer_manager.h[diff]
• third_party/blink/renderer/modules/media/audio/audio_renderer_mixer_manager_test.cc[diff]