Add browser tests for USB device add/remove events.

device/usb/usb_service.h

Index: device/usb/usb_service.h
diff --git a/device/usb/usb_service.h b/device/usb/usb_service.h
index 076d36377d0979f54a2b2e6a603644172ea77781..4655bfdbfa64d2ee640edbbde783f51aa10fc46c 100644
--- a/device/usb/usb_service.h
+++ b/device/usb/usb_service.h
@@ -55,8 +55,6 @@ class UsbService : public base::NonThreadSafe {
   void RemoveObserver(Observer* observer);
 
  protected:
-  friend struct base::DefaultDeleter<UsbService>;
-
   UsbService();
   virtual ~UsbService();
 
@@ -65,6 +63,9 @@ class UsbService : public base::NonThreadSafe {
 
   ObserverList<Observer, true> observer_list_;
 
+ private:
+  class Destroyer;
+
   DISALLOW_COPY_AND_ASSIGN(UsbService);
 };