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}
static ImageEventSender& errorEventSender() {
  DEFINE_STATIC_LOCAL(ImageEventSender, sender,
                      (ImageEventSender::create(EventTypeNames::error)));
  return sender;
}

static ImageEventSender& errorEventSender() {
  DEFINE_STATIC_LOCAL(ImageEventSender, sender,
                      (ImageEventSender::create(EventTypeNames::error)));
  return sender;
}
