Set up the infrastructure for Extension event metrics.

extensions/browser/api/usb/usb_event_router.cc

Index: extensions/browser/api/usb/usb_event_router.cc
diff --git a/extensions/browser/api/usb/usb_event_router.cc b/extensions/browser/api/usb/usb_event_router.cc
index bd171658e7c5a2b6f9adb628fbfba14757fa1835..895a3b65043711d0970d8235588ff51999cddd13 100644
--- a/extensions/browser/api/usb/usb_event_router.cc
+++ b/extensions/browser/api/usb/usb_event_router.cc
@@ -106,11 +106,11 @@ void UsbEventRouter::DispatchEvent(const std::string& event_name,
 
     scoped_ptr<Event> event;
     if (event_name == usb::OnDeviceAdded::kEventName) {
-      event.reset(new Event(usb::OnDeviceAdded::kEventName,
+      event.reset(new Event(events::UNKNOWN, usb::OnDeviceAdded::kEventName,
                             usb::OnDeviceAdded::Create(device_obj)));
     } else {
       DCHECK(event_name == usb::OnDeviceRemoved::kEventName);
-      event.reset(new Event(usb::OnDeviceRemoved::kEventName,
+      event.reset(new Event(events::UNKNOWN, usb::OnDeviceRemoved::kEventName,
                             usb::OnDeviceRemoved::Create(device_obj)));
     }