Roll WebRTC from fc6b87192db6 to ec9b281bbc14 (16 revisions)

https://webrtc.googlesource.com/src.git/+log/fc6b87192db6..ec9b281bbc14

2021-01-07 titovartem@google.com Add ability to specify random seed when creating built it network emulation
2021-01-07 danilchap@webrtc.org Introduce CopyOnWriteBuffer::MutableData() function
2021-01-07 alessiob@webrtc.org SSE2 and NEON kill switches for AGC2
2021-01-07 philipp.hancke@googlemail.com video_engine: fix logging
2021-01-07 perkj@webrtc.org Move setting of encoder bitrate allocation callback type to VideoSendStream
2021-01-07 chromium-webrtc-autoroll@webrtc-ci.iam.gserviceaccount.com Roll chromium_revision e2c7dc7720..25be8f6bfb (840733:840938)
2021-01-07 webrtc-version-updater@webrtc-ci.iam.gserviceaccount.com Update WebRTC code version (2021-01-07T04:03:27).
2021-01-06 chromium-webrtc-autoroll@webrtc-ci.iam.gserviceaccount.com Roll chromium_revision 6504b345eb..e2c7dc7720 (840589:840733)
2021-01-06 chromium-webrtc-autoroll@webrtc-ci.iam.gserviceaccount.com Roll chromium_revision 24b4cef393..6504b345eb (840473:840589)
2021-01-06 mbonadei@webrtc.org Add hta@ to rtc_base/ and api/ WATCHLISTS.
2021-01-06 mbonadei@webrtc.org Remove kwiberg@ from WATCHLIST.
2021-01-06 webrtc-version-updater@webrtc-ci.iam.gserviceaccount.com Update WebRTC code version (2021-01-06T04:02:29).
2021-01-06 chromium-webrtc-autoroll@webrtc-ci.iam.gserviceaccount.com Roll chromium_revision 49b7dc5e37..24b4cef393 (840329:840473)
2021-01-05 chromium-webrtc-autoroll@webrtc-ci.iam.gserviceaccount.com Roll chromium_revision c87d451712..49b7dc5e37 (840164:840329)
2021-01-05 chromium-webrtc-autoroll@webrtc-ci.iam.gserviceaccount.com Roll chromium_revision 4ae118108a..c87d451712 (840025:840164)
2021-01-05 terelius@webrtc.org Add static constexpr type to RTC event log events.

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/webrtc-chromium-autoroll
Please CC webrtc-chromium-sheriffs-robots@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/master/autoroll/README.md

Bug: None
Tbr: webrtc-chromium-sheriffs-robots@google.com
Change-Id: I4db24386b3e5cbba55794fe608293ff33f19e904
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2615982
Reviewed-by: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Commit-Queue: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#841208}