Per spec, "auto" represents the scrolling element of the document.
However, the scrolling element might be changed, based on the layout, in
quirks mode. Besides, the content of the root scroll frame is the root
element, instead of the scrolling element (e.g. body element) in both
standard and quirks modes. So now we define a special type, Scroller, to
represent the source of scroll-timeline, and use its |mType| to decide
which scroll frame we would like to use. In addition, hope this change let
us easier to implement nearest scroller.

Note: for auto scroller, we register this ScrollTimeline to the root
element, in both modes. Once we expose ScrollTimeline interface to the
script, we can rely on the |mType| of Scroller to return the correct
source element, whether it is scrolling element or not.