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As we're going to use DesktopStreamRegistry for both desktopCapture
and tabCapture registry, it's important to let getUserMedia() to be
called with corresponding source type, 'desktop' or 'tab', which will
lead to different UI indicator.
This cl is to add the source type at adding/verifying request to
the registry.

Bug: 831722
Change-Id: If53022ec35c3c3b7993e41149e381b96b3e48877
Reviewed-on: https://chromium-review.googlesource.com/1149192Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Commit-Queue: Weiyong Yao <braveyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581108}