webrtc-internals: add deprecation warning for offerExtmapAllowMixed

increasing visibility for developers

BUG=chromium:1199623

Change-Id: Ia59fd423cbd196f1223cf510587e03b34d347ace
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2849111
Reviewed-by: Henrik Boström <hbos@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/master@{#876768}

content/browser/webrtc/resources/webrtc_internals.css

@@ -132,3 +132,7 @@ th {
 details[open] details summary {
     background-color: rgb(220, 220, 220);
 }
+
+.peerconnection-deprecations {
+  font-weight: bold;
+}

content/browser/webrtc/resources/webrtc_internals.js

@@ -327,6 +327,22 @@ function addPeerConnection(data) {
   }
   peerConnectionElement.appendChild(p);
 
+  // Show deprecation notices as a list.
+  const deprecationNotices = document.createElement('ul');
+  deprecationNotices.className = 'peerconnection-deprecations';
+  // Note: data.rtcConfiguration is not in JSON format.
+  if (data.rtcConfiguration.indexOf('extmapAllowMixed: false') !== -1) {
+    // Hard deprecation, setting "false" will no longer work.
+    appendChildWithText(deprecationNotices, 'li',
+      'Note: The RTCPeerConnection offerAllowExtmapMixed ' +
+      'option is a non-standard feature. This feature will be removed ' +
+      'in M93 (Canary: July 15, 2021; Stable: August 24, 2021). For ' +
+      'interoperability with legacy WebRTC versions that throw errors ' +
+      'when attempting to parse the a=extmap-allow-mixed line in the ' +
+      'SDP remove the line from the SDP during signalling.');
+  }
+  peerConnectionElement.appendChild(deprecationNotices);
+
   return peerConnectionElement;
 }