Roll WebRTC from b6e840c0368f to 6e7167456b5e (4 revisions)

https://webrtc.googlesource.com/src.git/+log/b6e840c0368f..6e7167456b5e

2020-12-02 sprang@webrtc.org Adds experimental libvpx VP9 speed settings.
2020-12-02 landrey@webrtc.org Set default max_threads_count to DVQA injection helper
2020-12-02 landrey@webrtc.org Fix DVQA cpu usage when Stop is called multiple times
2020-12-02 landrey@webrtc.org Revert "Ignore frames that are comming to DVQA after Stop is called"

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: I5a69590e02e6879cf4f521ffac0bc2a755b56aa6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2570228Reviewed-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@{#833116}

DEPS

@@ -1551,7 +1551,7 @@ deps = {
   },
 
   'src/third_party/webrtc':
-    Var('webrtc_git') + '/src.git' + '@' + 'b6e840c0368f94366ad7f2c554d84e1d14f36ad4',
+    Var('webrtc_git') + '/src.git' + '@' + '6e7167456b5eba36c7985d6a74f1d191958d4e0f',
 
   'src/third_party/libgifcodec':
      Var('skia_git') + '/libgifcodec' + '@'+  Var('libgifcodec_revision'),