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::setImage(ImageResourceContent* newImage) {
  setImageWithoutConsideringPendingLoadEvent(newImage);

  updatedHasPendingEvent();
}

void ImageLoader::setImage(ImageResourceContent* newImage) {
  setImageWithoutConsideringPendingLoadEvent(newImage);

  updatedHasPendingEvent();
}
