Roll src/third_party/webrtc 4518a20e14ec..420ad1af1ef1 (1 commits)

https://webrtc.googlesource.com/src.git/+log/4518a20e14ec..420ad1af1ef1

git log 4518a20e14ec..420ad1af1ef1 --date=short --first-parent --format='%ad %ae %s'
2020-03-04 eshr@google.com Fix video_loopback crash when selecting all streams

Created with:
  gclient setdep -r src/third_party/webrtc@420ad1af1ef1

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: I515a58750ce9138c9cb7f101a12e98d0accf2616
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2087837Reviewed-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@{#746802}

DEPS

@@ -1440,7 +1440,7 @@ deps = {
     Var('chromium_git') + '/external/github.com/gpuweb/cts.git' + '@' + 'ec18cc3262922e7dcdbe70243c6f40606f979144',
 
   'src/third_party/webrtc':
-    Var('webrtc_git') + '/src.git' + '@' + '4518a20e14ecb3415c52aecf132d9bc83edaa9c4',
+    Var('webrtc_git') + '/src.git' + '@' + '420ad1af1ef1a58c8f366a969a4ff4d1e496171a',
 
   'src/third_party/libgifcodec':
      Var('skia_git') + '/libgifcodec' + '@'+  Var('libgifcodec_revision'),