Move ImageLoader timer to frame-specific TaskRunnerTimer.

Move ImageLoader timer m_derefElementTimer to frame-specific TaskRunnerTimer.
This associates it with the frame's Networking timer task queue.

BUG=624694

Review-Url: https://codereview.chromium.org/2642103002
Cr-Commit-Position: refs/heads/master@{#444927}
 ImageLoader::ImageLoader(Element* element)
     : m_element(element),
      m_derefElementTimer(TaskRunnerHelper::get(TaskType::Networking,
                                                element->document().frame()),
                          this,
                          &ImageLoader::timerFired),
       m_hasPendingLoadEvent(false),
       m_hasPendingErrorEvent(false),
       m_imageComplete(true),
      m_loadingImageDocument(false),
      m_elementIsProtected(false),
      m_suppressErrorEvents(false) {
  RESOURCE_LOADING_DVLOG(1) << "new ImageLoader " << this;
}

 ImageLoader::ImageLoader(Element* element)
     : m_element(element),
      m_derefElementTimer(this, &ImageLoader::timerFired),
       m_hasPendingLoadEvent(false),
       m_hasPendingErrorEvent(false),
       m_imageComplete(true),
      m_loadingImageDocument(false),
      m_elementIsProtected(false),
      m_suppressErrorEvents(false) {
  RESOURCE_LOADING_DVLOG(1) << "new ImageLoader " << this;
}
