GitLab

Add a volume change source param that is passed through to observers, to
make it easier to determine when to display UX feedback. Also, remove
weak functions since this API is required now.

Bug: internal b/121396128
Change-Id: Ie56168108810467bb3dcb8fb47efd1fc80eaca1f
Reviewed-on: https://chromium-review.googlesource.com/c/1493387Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Kenneth MacKay <kmackay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636703}