Updated comments for unit tests to validate iOS audio session isInterrupted flag does not get reset correctly.

BUG=webrtc:7605

Review-Url: https://codereview.webrtc.org/2886183002
Cr-Commit-Position: refs/heads/master@{#18188}

webrtc/modules/audio_device/ios/audio_device_unittest_ios.mm

@@ -825,6 +825,24 @@ TEST_F(AudioDeviceTest, DISABLED_MeasureLoopbackLatency) {
   latency_audio_stream->PrintResults();
 }
 
+// Verifies that the AudioDeviceIOS is_interrupted_ flag is reset correctly
+// after an iOS AVAudioSessionInterruptionTypeEnded notification event.
+// AudioDeviceIOS listens to RTCAudioSession interrupted notifications by:
+// - In AudioDeviceIOS.InitPlayOrRecord registers its audio_session_observer_
+//   callback with RTCAudioSession's delegate list.
+// - When RTCAudioSession receives an iOS audio interrupted notification, it
+//   passes the notification to callbacks in its delegate list which sets
+//   AudioDeviceIOS's is_interrupted_ flag to true.
+// - When AudioDeviceIOS.ShutdownPlayOrRecord is called, its
+//   audio_session_observer_ callback is removed from RTCAudioSessions's
+//   delegate list.
+//   So if RTCAudioSession receives an iOS end audio interruption notification,
+//   AudioDeviceIOS is not notified as its callback is not in RTCAudioSession's
+//   delegate list. This causes AudioDeviceIOS's is_interrupted_ flag to be in
+//   the wrong (true) state and the audio session will ignore audio changes.
+// As RTCAudioSession keeps its own interrupted state, the fix is to initialize
+// AudioDeviceIOS's is_interrupted_ flag to RTCAudioSession's isInterrupted
+// flag in AudioDeviceIOS.InitPlayOrRecord.
 TEST_F(AudioDeviceTest, testInterruptedAudioSession) {
   RTCAudioSession *session = [RTCAudioSession sharedInstance];
   std::unique_ptr<webrtc::AudioDeviceIOS> audio_device;