Revert of Delete SignalThread class. (patchset #20 id:380001 of https://codereview.webrtc.org/2915253002/ )

Reason for revert:
Seems to be causing new crashes, possibly because of changes to the "Destroy(false)" behavior. Will re-land after investigating these crashes more and addressing the root cause.

Original issue's description:
> Delete SignalThread class.
>
> Rewrite AsyncResolver to use PlatformThread directly, not
> SignalThread, and update includes of peerconnection client to not
> depend on signalthread.h.
>
> BUG=webrtc:6424,webrtc:7723
>
> Review-Url: https://codereview.webrtc.org/2915253002
> Cr-Commit-Position: refs/heads/master@{#18833}
> Committed: https://chromium.googlesource.com/external/webrtc/+/bc8feda1db02b2a9b501e4aa43926ca7e861b638

TBR=tommi@webrtc.org,kwiberg@webrtc.org,nisse@webrtc.org
NOPRESUBMIT=true
NOTRY=true
BUG=webrtc:6424,webrtc:7723

Review-Url: https://codereview.webrtc.org/2979733002
Cr-Commit-Position: refs/heads/master@{#18980}
11 files changed
tree: c0bca2e5712f05a11b705017ff7647491ebde0cd
  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