Fix teardown of stale AudioStreamMonitor poll callbacks.

content/browser/media/audio_stream_monitor.cc

Index: content/browser/media/audio_stream_monitor.cc
diff --git a/content/browser/media/audio_stream_monitor.cc b/content/browser/media/audio_stream_monitor.cc
index f2782a6be18f5ebe2fccfa2fff1d87c686a816e6..61f55f77a9d0746e179865ba0d08edfd2ea39155 100644
--- a/content/browser/media/audio_stream_monitor.cc
+++ b/content/browser/media/audio_stream_monitor.cc
@@ -65,6 +65,29 @@ bool AudioStreamMonitor::IsCurrentlyAudible() const {
   return is_audible_;
 }
 
+void AudioStreamMonitor::RenderProcessGone(int render_process_id) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+
+  // Note: It's possible for the RenderProcessHost and WebContents (and thus
+  // this class) to survive the death of the render process and subsequently be
+  // reused. During this period StartStopMonitoringHelper() will be unable to
+  // lookup the WebContents using the now-dead |render_frame_id|. We must thus
+  // have this secondary mechanism for clearing stale callbacks.
+
+  for (auto it = poll_callbacks_.begin(); it != poll_callbacks_.end();) {
+    if (it->first.first == render_process_id) {
+      it = poll_callbacks_.erase(it);
+      if (!power_level_monitoring_available())
+        OnStreamRemoved();
+    } else {
+      ++it;
+    }
+  }
+
+  if (poll_callbacks_.empty())
+    poll_timer_.Stop();
+}
+
 // static
 void AudioStreamMonitor::StartMonitoringStream(
     int render_process_id,
@@ -147,10 +170,12 @@ void AudioStreamMonitor::StopMonitoringStreamOnUIThread(int render_process_id,
                                                         int stream_id) {
   DCHECK(thread_checker_.CalledOnValidThread());
 
-  const StreamID qualified_id(render_process_id, stream_id);
-  DCHECK(poll_callbacks_.find(qualified_id) != poll_callbacks_.end());
-  poll_callbacks_.erase(qualified_id);
+  // In the event of render process death, these may have already been cleared.
+  auto it = poll_callbacks_.find(StreamID(render_process_id, stream_id));
+  if (it == poll_callbacks_.end())
+    return;
 
+  poll_callbacks_.erase(it);
   if (poll_callbacks_.empty())
     poll_timer_.Stop();
 }