We can now set the H264 profile, level and multithreaded encoding when we get the updated library. If we don't bump the API version in the struct, we will get the same behaviour as before.

This patch allows WebRTC to configure the OpenH264's encoder H264
profile and level, as well as multithreaded encoding. By default this is
turned off, and we can turn it on once we validate the library update.

The multithreaded encoding saw a ~15% performance improvement with
Chrome. See this bug for details:
https://bugs.chromium.org/p/chromium/issues/detail?id=1397287

Backed out for causing mda failures in test_peerConnection_basicH264Video.html

• Backout link
• Push with failures
• Failure Log
• Failure line: Hit MOZ_CRASH(IPDL error: "Error deserializing 'GMPVideoCodec'". abort()ing as a result.) at /builds/worker/checkouts/gecko/ipc/glue/ProtocolUtils.cpp:175
  TEST-UNEXPECTED-FAIL | dom/media/webrtc/tests/mochitests/test_peerConnection_basicH264Video.html | Error in test execution: Error: Checking stats for track {8ef78250-509e-444a-9e20-102cd5098732} timed out after 30000 ms _waitForRtpFlow@https://example.com/tests/dom/media/webrtc/tests/mochitests/pc.js:1809:11 ... async*waitForOutboundRtpFlow@https://example.com/tests/dom/media/webrtc/tests/mochitests/pc.js:1835:17 ... waitForMediaFlow/<@https://

There is an r+ patch which didn't land and no activity in this bug for 2 weeks.
:aosmond, could you have a look please?
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