Revert of Adding support for Unified Plan offer/answer negotiation. (patchset #9 id:500001 of https://codereview.webrtc.org/2991693002/ )

Reason for revert:
BUG=webrtc:8108: breaks Clang build.

Original issue's description:
> Adding support for Unified Plan offer/answer negotiation to the mediasession layer.
>
> This layer takes in a simplified "options" struct and the current local description,
> and generates a new offer/answer. Previously the options struct assumed there would
> only be one media description per media type (audio/video), but it now supports
> N number of audio/video descriptions.
>
> The |add_legacy_stream| options is removed from the mediasession.cc/.h
> in this CL.
>
> The next step is to add the ability for PeerConnection/WebRtcSession to create
> "options" to represent multiple RtpTransceivers, and apply the Unified Plan
> descriptions correctly. Right now, only Plan B descriptions will be
> generated in unit tests.
>
> BUG=chromium:465349
>
> Review-Url: https://codereview.webrtc.org/2991693002
> Cr-Commit-Position: refs/heads/master@{#19343}
> Committed: https://chromium.googlesource.com/external/webrtc/+/a77e6bbd30276bdc5b30f2cbc1e92ca181ae76f0

TBR=deadbeef@webrtc.org,zhihuang@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=chromium:465349

Review-Url: https://codereview.webrtc.org/3001083002
Cr-Commit-Position: refs/heads/master@{#19384}
8 files changed
tree: 90ee4cfec759a2bafdcb7b99a030bdb27187b99c
  1. build_overrides/
  2. data/
  3. infra/
  4. resources/
  5. tools_webrtc/
  6. webrtc/
  7. .clang-format
  8. .git-blame-ignore-revs
  9. .gitignore
  10. .gn
  11. AUTHORS
  12. BUILD.gn
  13. check_root_dir.py
  14. cleanup_links.py
  15. CODE_OF_CONDUCT.md
  16. codereview.settings
  17. DEPS
  18. LICENSE
  19. license_template.txt
  20. LICENSE_THIRD_PARTY
  21. OWNERS
  22. PATENTS
  23. PRESUBMIT.py
  24. pylintrc
  25. README.md
  26. WATCHLISTS
README.md

WebRTC is a free, open software project that provides browsers and mobile applications with Real-Time Communications (RTC) capabilities via simple APIs. The WebRTC components have been optimized to best serve this purpose.

Our mission: To enable rich, high-quality RTC applications to be developed for the browser, mobile platforms, and IoT devices, and allow them all to communicate via a common set of protocols.

The WebRTC initiative is a project supported by Google, Mozilla and Opera, amongst others.

Development

See http://www.webrtc.org/native-code/development for instructions on how to get started developing with the native code.

More info