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}
DEFINE_TRACE(ImageLoader) {
  visitor->trace(m_image);
  visitor->trace(m_imageResourceForImageDocument);
  visitor->trace(m_element);
}

DEFINE_TRACE(ImageLoader) {
  visitor->trace(m_image);
  visitor->trace(m_imageResourceForImageDocument);
  visitor->trace(m_element);
}
