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/1605513002

Cr-Commit-Position: refs/heads/master@{#370219}
6 files changed