Roll WebRTC from 8592111e458d to 2157c5d256c2 (12 revisions)

https://webrtc.googlesource.com/src.git/+log/8592111e458d..2157c5d256c2

2020-12-18 webrtc-version-updater@webrtc-ci.iam.gserviceaccount.com Update WebRTC code version (2020-12-18T04:04:58).
2020-12-17 handellm@webrtc.org RtpTransceiverInterface: introduce HeaderExtensionsNegotiated.
2020-12-17 chromium-webrtc-autoroll@webrtc-ci.iam.gserviceaccount.com Roll chromium_revision 1a8a708c..31e5ea78 (838088:838188)
2020-12-17 hta@webrtc.org Declare the Clone operator of SessionDescriptionInterface as const.
2020-12-17 chromium-webrtc-autoroll@webrtc-ci.iam.gserviceaccount.com Roll chromium_revision c8be2f62..1a8a708c (837967:838088)
2020-12-17 mbonadei@webrtc.org Make RTC_OBJC_EXPORT respect is_component_build.
2020-12-17 ivoc@webrtc.org Fix for overflow in iLBC code.
2020-12-17 ivoc@webrtc.org Use low latency mode on Android O and later.
2020-12-17 philipp.hancke@googlemail.com sdp: remove extra check for attribute type when parsing fingerprint
2020-12-17 chromium-webrtc-autoroll@webrtc-ci.iam.gserviceaccount.com Roll chromium_revision 6370178a..c8be2f62 (837763:837967)
2020-12-17 webrtc-version-updater@webrtc-ci.iam.gserviceaccount.com Update WebRTC code version (2020-12-17T07:06:37).
2020-12-17 jianj@google.com Decide Av1 num of threads on frame size and available cores.

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: chromium:1051821,chromium:1158070,chromium:1159620
Tbr: webrtc-chromium-sheriffs-robots@google.com
Change-Id: If073586bcef31634763c4ce841302fa0f1980b32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2598358Reviewed-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@{#838441}

DEPS

@@ -1514,7 +1514,7 @@ deps = {
     Var('chromium_git') + '/external/github.com/gpuweb/cts.git' + '@' + '3c2fe3888658d82b47ca831d59a2e07579619c2d',
 
   'src/third_party/webrtc':
-    Var('webrtc_git') + '/src.git' + '@' + '8592111e458d253e53806db65d23d21bfe9d9db4',
+    Var('webrtc_git') + '/src.git' + '@' + '2157c5d256c2c32995def0a67a448dd1928edef5',
 
   'src/third_party/libgifcodec':
      Var('skia_git') + '/libgifcodec' + '@'+  Var('libgifcodec_revision'),