Adds some code to help isolate where crash is happening

mojo/message_pump/message_pump_mojo.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_MESSAGE_PUMP_MESSAGE_PUMP_MOJO_H_
 #define MOJO_MESSAGE_PUMP_MESSAGE_PUMP_MOJO_H_
 
 #include <map>
 
 #include "base/macros.h"
@@ skipping 32 matching lines @@
   static scoped_ptr<base::MessagePump> Create();
 
   // Returns the MessagePumpMojo instance of the current thread, if it exists.
   static MessagePumpMojo* current();
 
   static bool IsCurrent() { return !!current(); }
 
   // Registers a MessagePumpMojoHandler for the specified handle. Only one
   // handler can be registered for a specified handle.
   // NOTE: a value of 0 for |deadline| indicates an indefinite timeout.
-  void AddHandler(MessagePumpMojoHandler* handler,
+  void AddHandler(int location,
+                  MessagePumpMojoHandler* handler,
                   const Handle& handle,
                   MojoHandleSignals wait_signals,
                   base::TimeTicks deadline);
 
   void RemoveHandler(const Handle& handle);
 
   void AddObserver(Observer*);
   void RemoveObserver(Observer*);
 
   // MessagePump:
   void Run(Delegate* delegate) override;
   void Quit() override;
   void ScheduleWork() override;
   void ScheduleDelayedWork(const base::TimeTicks& delayed_work_time) override;
 
  private:
   struct RunState;
   struct WaitState;
 
   // Contains the data needed to track a request to AddHandler().
   struct Handler {
-    Handler() : handler(NULL), wait_signals(MOJO_HANDLE_SIGNAL_NONE), id(0) {}
+    Handler()
+        : location(0),
+          handler(nullptr),
+          wait_signals(MOJO_HANDLE_SIGNAL_NONE),
+          id(0) {}
 
+    int location;
     MessagePumpMojoHandler* handler;
     MojoHandleSignals wait_signals;
     base::TimeTicks deadline;
     // See description of |MessagePumpMojo::next_handler_id_| for details.
     int id;
   };
 
   typedef std::map<Handle, Handler> HandleToHandler;
 
   // Implementation of Run().
@@ skipping 40 matching lines @@
 
   base::ObserverList<Observer> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(MessagePumpMojo);
 };
 
 }  // namespace common
 }  // namespace mojo
 
 #endif  // MOJO_MESSAGE_PUMP_MESSAGE_PUMP_MOJO_H_