Prevent a race condition with MidiHost IPC sending.

content/browser/media/midi_host.cc

Index: content/browser/media/midi_host.cc
diff --git a/content/browser/media/midi_host.cc b/content/browser/media/midi_host.cc
index 1e6de1e234f1b6b5eebdf01a97c68de307b3a139..fe8e2fbdcfb47a64b2c3c93d63874a7e6bc44c6c 100644
--- a/content/browser/media/midi_host.cc
+++ b/content/browser/media/midi_host.cc
@@ -59,10 +59,19 @@ MidiHost::MidiHost(int renderer_process_id,
   DCHECK(midi_manager_);
 }
 
-MidiHost::~MidiHost() {
-  // Close an open session, or abort opening a session.
-  if (is_session_requested_ && midi_manager_)
+MidiHost::~MidiHost() {}

[Adam Goode, 2016/01/12 17:36:45]

Suggest (C++11 style):

MidiHost::~MidiHost() = default;

[Oliver Chang, 2016/01/12 17:48:55]

Done.

+
+void MidiHost::OnChannelClosing() {
+  // If we get here the MidiHost is going to be destroyed soon. Prevent any
+  // subsequent calls from MidiManager by closing our session.
+  // If Send() is called from a different thread (e.g. a separate thread owned
+  // by the MidiManager implementation), it will get posted to the IO thread.
+  // There is a race condition here if our refcount is 0 and we're about to or
+  // have already entered OnDestruct().
+  if (is_session_requested_ && midi_manager_) {
     midi_manager_->EndSession(this);
+    is_session_requested_ = false;
+  }
 }
 
 void MidiHost::OnDestruct() const {