Revert of Delete class SSRCDatabase, and its global ssrc registry. (patchset #20 id:370001 of https://codereview.webrtc.org/2644303002/ )

Reason for revert:
Breaks webrtc_perf_tests reliably:
https://build.chromium.org/p/client.webrtc.perf/builders/Android32%20Tests%20%28L%20Nexus5%29/builds/1780
https://build.chromium.org/p/client.webrtc.perf/builders/Android32%20Tests%20%28L%20Nexus4%29/builds/178

We're actively working on getting a quick version of webrtc_perf_tests up on the trybots again to prevent breakages like this: https://bugs.chromium.org/p/webrtc/issues/detail?id=7101

Original issue's description:
> Delete class SSRCDatabase, and its global ssrc registry,
> and the method RTPSender::GenerateNewSSRC.
>
> It's now mandatory for higher layers to call SetSSRC, RTPSender
> no longer allocates any ssrc by default.
>
> BUG=webrtc:4306,webrtc:6887
>
> Review-Url: https://codereview.webrtc.org/2644303002
> Cr-Commit-Position: refs/heads/master@{#16670}
> Committed: https://chromium.googlesource.com/external/webrtc/+/b78d4d13835f628e722a57abae2bf06ba3655921

TBR=solenberg@webrtc.org,stefan@webrtc.org,danilchap@webrtc.org,ivoc@webrtc.org,nisse@webrtc.org
NOTRY=True
BUG=webrtc:4306,webrtc:6887

Review-Url: https://codereview.webrtc.org/2700413002
Cr-Commit-Position: refs/heads/master@{#16693}
13 files changed
tree: 186b51a5150df2d9ebc991d0fcfd5a226e14907a
  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. codereview.settings
  16. DEPS
  17. LICENSE
  18. license_template.txt
  19. LICENSE_THIRD_PARTY
  20. OWNERS
  21. PATENTS
  22. PRESUBMIT.py
  23. pylintrc
  24. README.md
  25. 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. This page is maintained by the Google Chrome team.

Development

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

More info