Address Peter's comments from past review

Source/modules/mediastream/MediaDevicesRequest.cpp

 /*
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 47 matching lines @@
 {
     if (ExecutionContext* context = executionContext()) {
         return toDocument(context);
     }
 
     return 0;
 }
 
 ScriptPromise MediaDevicesRequest::start()
 {
-    if (m_controller) {
-        m_resolver->keepAliveWhilePending();
-        m_controller->requestMediaDevices(this);
-        return m_resolver->promise();
-    }
-
-    return ScriptPromise::rejectWithDOMException(m_resolver->scriptState(), DOMException::create(NotSupportedError, "No media device controller available"));
+    ASSERT(m_controller);
+    m_resolver->keepAliveWhilePending();
+    m_controller->requestMediaDevices(this);
+    return m_resolver->promise();
 }
 
 void MediaDevicesRequest::succeed(const MediaDeviceInfoVector& mediaDevices)
 {
     if (!executionContext() || !m_resolver)
         return;
 
     m_resolver->resolve(mediaDevices);
 }
 
 void MediaDevicesRequest::stop()
 {
     m_controller.clear();
     m_resolver.clear();
 }
 
 DEFINE_TRACE(MediaDevicesRequest)
 {
     visitor->trace(m_controller);
     visitor->trace(m_resolver);
     ActiveDOMObject::trace(visitor);
 }
 
 } // namespace blink