Add a hang monitor for Pepper plugins

ppapi/proxy/dispatcher.cc

Index: ppapi/proxy/dispatcher.cc
diff --git a/ppapi/proxy/dispatcher.cc b/ppapi/proxy/dispatcher.cc
index a2a1d7137a38f2caf5f180d28abe2a795cca3eaf..39b1a063f94e64e48915ae93426f347ada5c4393 100644
--- a/ppapi/proxy/dispatcher.cc
+++ b/ppapi/proxy/dispatcher.cc
@@ -50,6 +50,9 @@ base::MessageLoopProxy* Dispatcher::GetIPCMessageLoop() {
 
 void Dispatcher::AddIOThreadMessageFilter(
     IPC::ChannelProxy::MessageFilter* filter) {
+  // Our filter is refcounted. The channel will call the destruct method on the
+  // filter when the channel is done with it, so the corresponding Release()
+  // happens there.
   channel()->AddFilter(filter);
 }