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Reason for revert:
Re-landing with fixed tests.

Original rollback description:
> Revert of Remove MockConstraints (patchset #1 id:1 of https://codereview.chromium.org/1605513002/ )
>
> Reason for revert:
> Appears to be causing fast/mediastream/RTCPeerConnection.html to fail across all bots.

Original issue's description:

Remove MockConstraints

The purpose of the MockConstraints class was to verify that only legal
constraint names were passed. This is now done inside Blink
in MediaConstraintsImpl.cpp, so this class isn't needed any more.

BUG=543997

Review URL: https://codereview.chromium.org/1614733002

Cr-Commit-Position: refs/heads/master@{#370729}