Move gtest-parallel from WebRTC into separate repo.

With gtest-parallel living in its own repo, it will be possible to
DEPS it in independently of the WebRTC source code (which also
makes it more reusable by other projects).

This change simply adds all the files in
https://chromium.googlesource.com/external/webrtc/+/master/third_party/gtest-parallel
(which will be removed as soon this lands and has been properly
added to DEPS).

A codereview.settings file is also added.

BUG=webrtc:5006
6 files changed