Oilpan: Prepare to move ImageLoader and its subclasses to Oilpan heap.

- We need to add Persistent<Element> in ImageLoader in order to
  follow the current behavior by manual ref/deref.
  Also, the manual ref/deref is replaced with RefPtr<Element>.

- ImageLoader was the last off-heap user of EventSender<T>. Now all of
  EventSender<T> users are on-heap. We can trace EventSender::m_dispatchSoonList
  and m_dispatchingList.

- HTMLImageElement and SVGImageElement had ImageLoader as part objects. This CL
  makes them OwnPtr / Member because using on-heap objects as part objects are
  dangerous.

- Add HTMLImageElement::create and SVGImageElement::create.

BUG=357163
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/320253002

git-svn-id: svn://svn.chromium.org/blink/trunk@175876 bbb929c8-8fbe-4397-9dbb-9b2b20218538