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}
bool ImageLoader::shouldLoadImmediately(const KURL& url) const {
  if (!url.isNull()) {
    Resource* resource = memoryCache()->resourceForURL(
        url, m_element->document().fetcher()->getCacheIdentifier());
    if (resource && !resource->errorOccurred())
      return true;
  }
  return (isHTMLObjectElement(m_element) || isHTMLEmbedElement(m_element));
}

bool ImageLoader::shouldLoadImmediately(const KURL& url) const {
  if (!url.isNull()) {
    Resource* resource = memoryCache()->resourceForURL(
        url, m_element->document().fetcher()->getCacheIdentifier());
    if (resource && !resource->errorOccurred())
      return true;
  }
  return (isHTMLObjectElement(m_element) || isHTMLEmbedElement(m_element));
}
