[webrtc-pc] new commits pushed by burnburn

The following commits were just pushed by burnburn to https://github.com/w3c/webrtc-pc:

* Removing WebIDL defaults for various RTP parameters.

Fixes #1173.

`setParameters` is only expected to be called with a result of
`getParameters` (enforced by the `transactionId` field). So, these
WebIDL defaults serve no purpose. If a parameter has a default value,
`getParameters` will have already returned it.
  by Taylor Brandstetter
https://github.com/w3c/webrtc-pc/commit/4c586b79097214e0391fc5e23aee012299692e66

* Responding to Bernard's comments.
  by Taylor Brandstetter
https://github.com/w3c/webrtc-pc/commit/a474b9249cb02b1eb0f4305e3a2d2f49b2e3209a

* Add "Read-only parameter" to codec parameter descriptions.
  by Taylor Brandstetter
https://github.com/w3c/webrtc-pc/commit/8114bd76f4831b5d9121cc0330013f1812330762

* Merge pull request #1226 from taylor-b/issue_1173_rtpparameters_default_values

Removing WebIDL defaults for various RTP parameters.
  by Daniel Burnett
https://github.com/w3c/webrtc-pc/commit/77ff0475c916e8eb9db7b5585762c1fa3cbf6782