GitLab

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
>
> Committed: https://crrev.com/dc42f8b18dbb9c8fec81e3b7a10ab0186ce36553
> Cr-Commit-Position: refs/heads/master@{#370219}

TBR=jochen@chromium.org,tommi@chromium.org,hta@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=543997

Review URL: https://codereview.chromium.org/1608073002

Cr-Commit-Position: refs/heads/master@{#370238}