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The original code has unnecessary DCHECKs that will cause crashes when
audio/video config got changed more than once. This CL removed them and
also refactor the config changes flow.

Bug: 1111702
Change-Id: Id3a7ba39cc5a5d433f4a0a791f10c75baa43ed79
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2332082Reviewed-by: Yuri Wiitala <miu@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Commit-Queue: Chih-Hsuan Kuo <chkuo@google.com>
Auto-Submit: Chih-Hsuan Kuo <chkuo@google.com>
Cr-Commit-Position: refs/heads/master@{#794784}