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}
void ImageLoader::dispatchPendingErrorEvent() {
  if (!m_hasPendingErrorEvent)
    return;
  m_hasPendingErrorEvent = false;

  if (element()->document().frame())
    element()->dispatchEvent(Event::create(EventTypeNames::error));

  updatedHasPendingEvent();
}

void ImageLoader::dispatchPendingErrorEvent() {
  if (!m_hasPendingErrorEvent)
    return;
  m_hasPendingErrorEvent = false;

  if (element()->document().frame())
    element()->dispatchEvent(Event::create(EventTypeNames::error));

  updatedHasPendingEvent();
}
