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This CL adds CombinedDesktopMediaList class and its associated unit tests. It intends to support chrome tab capture for JavaScript API chooseDesktopMedia().

Design document is in
https://docs.google.com/document/d/1zZOiayDzVkuIQ3twEdiE6S-LTXz9fWOGKjbMIsLTcAk/edit#heading=h.cetuprbl19fv

BUG=557222

Review URL: https://codereview.chromium.org/1622733002

Cr-Commit-Position: refs/heads/master@{#372228}