Revert "Revert webrtc unpinning."

This reverts commit ffea867.

Reason for revert: The WebRTC M71 branch has been fixed, so re-pin to it. Tested OK locally syncing deps and building with this revert. Here's the fixed WebRTC M71 branch: https://webrtc.googlesource.com/src/+log/branch-heads/71

Original change's description:
> Revert webrtc unpinning.
>
> Speculative fix that the webrtc m71 branch is out-of-sync with src.git
> and the cause of build failures.
>
> R=​govind@google.com, tommi@google.com
>
> Bug: 895012
> Change-Id: Ibd19f2371b6eda5d915ce4ebf9ba377e507ff0c6
> Reviewed-on: https://chromium-review.googlesource.com/c/1279345
> Reviewed-by: Krishna Govind <govind@chromium.org>
> Cr-Commit-Position: refs/branch-heads/3578@{#6}
> Cr-Branched-From: 4226ddf-refs/heads/master@{#599034}

TBR=tommi@chromium.org,mmoss@chromium.org,govind@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 895012
Change-Id: I4fe1be11eb845e36f752d05687bb7acf7af2b267
Reviewed-on: https://chromium-review.googlesource.com/c/1280666
Reviewed-by: Michael Moss <mmoss@chromium.org>
Reviewed-by: Tommi <tommi@chromium.org>
Reviewed-by: Krishna Govind <govind@chromium.org>
Reviewed-by: Henrik Grunell <grunell@chromium.org>
Cr-Commit-Position: refs/branch-heads/3578@{#25}
Cr-Branched-From: 4226ddf-refs/heads/master@{#599034}

DEPS

@@ -1106,7 +1106,7 @@ deps = {
     Var('chromium_git') + '/external/khronosgroup/webgl.git' + '@' + '6d2f3f4cb8bac1f7c4a945c73d07a33df74f22f9',
 
   'src/third_party/webrtc':
-    Var('webrtc_git') + '/src.git' + '@' + 'fb226af64dadffcb3a90a9ba42c46164f320d43a',
+    Var('webrtc_git') + '/src.git' + '@' + 'refs/branch-heads/71',
 
   'src/third_party/xdg-utils': {
       'url': Var('chromium_git') + '/chromium/deps/xdg-utils.git' + '@' + 'd80274d5869b17b8c9067a1022e4416ee7ed5e0d',