Implement googContentType GetStats metric reported on receive side.

Reported per video stream as a string.

BUG=webrtc:8174

Review-Url: https://codereview.webrtc.org/3009793002
Cr-Original-Commit-Position: refs/heads/master@{#19667}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 2e1b40bdf63743b07388628707c4b8e349495807