webrtc: add third-party sdp module

Adds the third-party sdp module available from
  https://www.npmjs.com/package/sdp
in version 2.12.0 and commit
  958f22a94231e4c1ca651b4a7d53b0e32bb4a558
without tests or dependencies. This is going to be used by simulcast
tests and is generally useful in manipulating the SDP generated and
consumed by WebRTC.

See web-platform-tests/rfcs#46 for discussion
on how to add third-party javascript libraries.

webrtc/third_party/sdp/LICENSE

@@ -0,0 +1,19 @@
+Copyright (c) 2017 Philipp Hancke
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.