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MediaClientImpl needs to register itself as an observer of
VmCameraMicManager. Depending on whether the user has customized a flag
or not, MediaClientImpl is either constructed before or after
`UserSessionInitializer::OnUserSessionStarted(/*is_primary_user*/=true)`
is run. The reason seems to be that 1) MediaClientImpl is constructed in
a task queue; 2) Customizing a flag causes the browser to be restarted
after the user signs in, which changes the execution characteristics.

My last attempt (CL 2586286) to fix the registration does not work if
the user has customized flags. So, this CL makes VmCameraMicManager
a singleton which is always available so that MediaClientImpl can always
reliably do the registration in its ctor.

Bug: b/167491603
Change-Id: I2b73b59216851f0b8b2533120317e54c98382b45
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2589061
Commit-Queue: Jason Lin <lxj@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836941}