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This checkbox is set as checked by default at present. With some new
features, as getDisplayMedia and dynamic change capture source, it's
more reasonalbe to set it as unchecked by default. Also it will help
to count the requirement of audio capture during presenting.

Bug: 892201
Change-Id: I36c253bf1c862a31d99927be79c2072332cb1622
Reviewed-on: https://chromium-review.googlesource.com/c/1356713Reviewed-by: Qiang Chen <qiangchen@chromium.org>
Commit-Queue: Weiyong Yao <braveyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613266}