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This is part of the working for the dynamic change of screenshare
source feature. This cl is to add a ChangeDevice method to change the
audio/video source within the same session.

Note: Add a source/track is not supported in this cl.

Bug: 892201
Change-Id: I99e246456b735dbe66a82686745eb0a7c35a7ea1
Reviewed-on: https://chromium-review.googlesource.com/c/1330717Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Emircan Uysaler <emircan@chromium.org>
Commit-Queue: Weiyong Yao <braveyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611428}