Strip out stream counting from AudioRendererHost.

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 ba86d56a8f05f5b8c1ba9bc5d8e3f581b7d3f96a..3f1c09a4c4c8b6a8c40715c7c1cb4d760aa0499e 100644
--- a/content/browser/media/audio_stream_monitor.cc
+++ b/content/browser/media/audio_stream_monitor.cc
@@ -87,6 +87,12 @@ void AudioStreamMonitor::StartMonitoringHelper(
   if (!monitor)
     return;
 
+  RenderProcessHostImpl* render_process_host =

[DaleCurtis, 2017/01/30 21:12:19]

+ncarter for content/ review. Is it possible for this to ever return null if
l.85 returns a value?

[ncarter (slow), 2017/01/31 18:33:12]

This is safe, given the implementation of AudioStreamMonitorFromRenderFrame:

 WebContentsImpl* const web_contents =
      static_cast<WebContentsImpl*>(WebContents::FromRenderFrameHost(
          RenderFrameHost::FromID(render_process_id, render_frame_id)));

Since this does a lookup via RenderFrameHost::FromID(), we know there's a valid
RenderFrameHost* with this ID tuple, and a RenderFrameHost* keeps its
RenderProcessHost alive (via AddRoute/RemoveRoute).

However, this would be easier to see and verify if you just retained the
RenderFrameHost ptr used to lookup the WebContents, and called GetProcess() to
obtain the RPH. That would also be a touch faster.

[DaleCurtis, 2017/02/06 22:19:02]

Replaced all of this with a helper function. We can't use
RenderFrameHost::FromID here since we need to always send this even if the frame
host has gone away. The only way to do that seems to be using RPH::FromID().

+      static_cast<RenderProcessHostImpl*>(
+          RenderProcessHost::FromID(render_process_id));
+  if (!render_process_host)
+    return;
+  render_process_host->OnAudioStreamAdded();
   monitor->OnStreamAdded();
 
   if (!power_level_monitoring_available())
@@ -106,6 +112,12 @@ void AudioStreamMonitor::StopMonitoringHelper(int render_process_id,
   if (!monitor)
     return;
 
+  RenderProcessHostImpl* render_process_host =
+      static_cast<RenderProcessHostImpl*>(
+          RenderProcessHost::FromID(render_process_id));
+  if (!render_process_host)
+    return;
+  render_process_host->OnAudioStreamRemoved();

[ncarter (slow), 2017/01/31 18:33:12]

How confident are you that this perfectly balances the Added operation?

Does this function get executed if the process is killed (e.g. if you kill it
via task manager)?

[DaleCurtis, 2017/02/06 22:19:02]

Test added; it was incorrect before, but is correct now. The ordering of process
checks was wrong.

   monitor->OnStreamRemoved();
 
   if (!power_level_monitoring_available())