Roll src/third_party/webrtc 835dde17a0c9..976cc1ae1901 (4 commits)

https://webrtc.googlesource.com/src.git/+log/835dde17a0c9..976cc1ae1901

git log 835dde17a0c9..976cc1ae1901 --date=short --first-parent --format='%ad %ae %s'
2020-03-20 danilchap@webrtc.org Add AV1 encoder wrapper
2020-03-20 peah@webrtc.org Changed fft4g to be built as C++
2020-03-20 saza@webrtc.org Add empty placeholder target for rms_level.*
2020-03-20 brandtr@webrtc.org libvpx-vp8: Add settings struct to constructor.

Created with:
  gclient setdep -r src/third_party/webrtc@976cc1ae1901

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/+/master/autoroll/README.md

Bug: None
Tbr: webrtc-chromium-sheriffs-robots@google.com
Change-Id: Ie1105c30d78bebcb0fbaf6ab41d3f1db083c9a08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2112526Reviewed-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@{#752039}

DEPS

@@ -1448,7 +1448,7 @@ deps = {
     Var('chromium_git') + '/external/github.com/gpuweb/cts.git' + '@' + '84ee597cdeae08bb26e578fc66a35bcf35f633f4',
 
   'src/third_party/webrtc':
-    Var('webrtc_git') + '/src.git' + '@' + '835dde17a0c9acd06f73e81c4d4cf45960a36df1',
+    Var('webrtc_git') + '/src.git' + '@' + '976cc1ae190180738d44b3448499089b95d5865c',
 
   'src/third_party/libgifcodec':
      Var('skia_git') + '/libgifcodec' + '@'+  Var('libgifcodec_revision'),