GitLab

Replaced Stub with parent implementation stubs to make it easier
to add sounds; made the proxy explicit; renamed things for clarity.
Follow-up CL will remove accessibility_sound_delegate.h.

Bug: internal b/73383411
Test: manual
Change-Id: I95d6619f373024f66aed6204ee667585c7db0f76
Reviewed-on: https://chromium-review.googlesource.com/1166029Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Commit-Queue: Kevin Schoedel <kpschoedel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581931}