Add third_party/webrtc_overrides.

This is the first step in moving the overrides files from WebRTC to Chromium. See bug comment #65 for all steps.

The files are just copied from the WebRTC repo https://chromium.googlesource.com/external/webrtc/trunk/webrtc.git/+/master/overrides/ with updated comments and licence headers.

In these files, it's only the implementation files that depend on Chromium, thus removing WebRTC's dependency on Chromium when the move is complete.

BUG=468375

Review URL: https://codereview.chromium.org/1345873004

Cr-Commit-Position: refs/heads/master@{#351528}
7 files changed