Bug 1769985 add a dummy default audio output device to enumerate devices results when the first exposed output device is not the default r=jib

This is the simplest of the variations of the proposal at
https://github.com/w3c/mediacapture-output/issues/133#issuecomment-1271122304

Creating the dummy output in FilterExposedDevices(), rather than later in
the enumerateDevices() process, provides a "devicechange" event when an
exposed device becomes or ceases to be the default device.

Differential Revision: https://phabricator.services.mozilla.com/D159013

dom/media/MediaDevices.cpp

@@ -236,6 +236,8 @@ RefPtr<MediaDeviceSetRefCnt> MediaDevices::FilterExposedDevices(
     // they are exposed only when explicitly and individually allowed by the
     // user.
   }
+  bool outputIsDefault = true;  // First output is the default.
+  bool haveDefaultOutput = false;
   nsTHashSet<nsString> exposedMicrophoneGroupIds;
   for (const auto& device : aDevices) {
     switch (device->mKind) {
@@ -259,8 +261,21 @@ RefPtr<MediaDeviceSetRefCnt> MediaDevices::FilterExposedDevices(
             (!mExplicitlyGrantedAudioOutputRawIds.Contains(device->mRawID) &&
              // Assumes aDevices order has microphones before speakers.
              !exposedMicrophoneGroupIds.Contains(device->mRawGroupID))) {
+          outputIsDefault = false;
           continue;
         }
+        if (!haveDefaultOutput && !outputIsDefault) {
+          // Insert a virtual default device so that the first enumerated
+          // device is the default output.
+          RefPtr info = new AudioDeviceInfo(
+              nullptr, u""_ns, u""_ns, u""_ns, CUBEB_DEVICE_TYPE_OUTPUT,
+              CUBEB_DEVICE_STATE_ENABLED, CUBEB_DEVICE_PREF_ALL,
+              CUBEB_DEVICE_FMT_ALL, CUBEB_DEVICE_FMT_S16NE, 2, 44100, 44100,
+              44100, 128, 128);
+          exposed->AppendElement(
+              new MediaDevice(new MediaEngineFake(), info, u""_ns));
+        }
+        haveDefaultOutput = true;
         break;
       case MediaDeviceKind::EndGuard_:
         continue;

dom/media/MediaManager.cpp

@@ -2868,8 +2868,11 @@ RefPtr<LocalDeviceSetPromise> MediaManager::AnonymizeDevices(
             RefPtr anonymized = new LocalMediaDeviceSetRefCnt();
             for (const RefPtr<MediaDevice>& device : *rawDevices) {
               nsString id = device->mRawID;
-              nsContentUtils::AnonymizeId(id, aOriginKey);
-
+              // An empty id represents a virtual default device, for which
+              // the exposed deviceId is the empty string.
+              if (!id.IsEmpty()) {
+                nsContentUtils::AnonymizeId(id, aOriginKey);
+              }
               nsString groupId = device->mRawGroupID;
               // Use window id to salt group id in order to make it session
               // based as required by the spec. This does not provide unique