Makes AudioOutputProxy -> AudioOutputDispatcher reference weak.

media/audio/audio_output_dispatcher_impl.cc

Index: media/audio/audio_output_dispatcher_impl.cc
diff --git a/media/audio/audio_output_dispatcher_impl.cc b/media/audio/audio_output_dispatcher_impl.cc
index b538acde82cdb67b8a9117f0ba4b530091a2ddcc..1498bec35d93f8e9ce5484172ebbbf79b89cbf2d 100644
--- a/media/audio/audio_output_dispatcher_impl.cc
+++ b/media/audio/audio_output_dispatcher_impl.cc
@@ -34,13 +34,20 @@ AudioOutputDispatcherImpl::AudioOutputDispatcherImpl(
 AudioOutputDispatcherImpl::~AudioOutputDispatcherImpl() {
   CHECK(task_runner_->BelongsToCurrentThread());
 
+  // Stop all active streams.
+  for (auto iter = proxy_to_physical_map_.begin();
+       iter != proxy_to_physical_map_.end();) {
+    // Note: Stopping the stream will invalidate the iterator.
+    // Increment the iterator before stopping the stream.
+    AudioOutputProxy* stream_proxy = iter->first;
+    ++iter;
+    StopStream(stream_proxy);

[alokp, 2017/01/10 20:33:59]

Do you think it is worthwhile to trigger AudioSourceCallback::OnError to notify
the clients that playback will stop?

IMO it is probably not worth because the browser is already shutting down.

[DaleCurtis, 2017/01/10 20:54:41]

No, this should be fine. Also, just use a for each style loop instead?

[alokp, 2017/01/10 22:51:26]

I cannot use the for-each style because StopStream would invalidate the
iterator. Note the comment.

[DaleCurtis, 2017/01/10 22:59:05]

Hmm, for some reason I thought for-each is immune to iterator invalidation since
it's not really an iterator. I'll have to go back and check.

[alokp, 2017/01/10 23:31:53]

AFAICT none - for_each or c++11 range-based for-loop - do not guarantee iterator
to be valid:
http://en.cppreference.com/w/cpp/language/range-for
http://en.cppreference.com/w/cpp/algorithm/for_each

In fact the possible implementation provided in the documentation would clearly
not work.

+  }
+
   // Close all idle streams immediately.  The |close_timer_| will handle
   // invalidating any outstanding tasks upon its destruction.
   CloseAllIdleStreams();
 
-  // There must be no idle proxy streams.
-  CHECK_EQ(idle_proxies_, 0u);
-
   // There must be no active proxy streams.
   CHECK(proxy_to_physical_map_.empty());