Use Element instead of Node in DragState, also redo DragState struct
https://bugs.webkit.org/show_bug.cgi?id=116411

Reviewed by Sam Weinig.

The drag source is an element, so use RefPtr<Element> instead of RefPtr<Node>.

Also, the entire drag state is about dragging, so no need for the word "drag" in the
name of its members. Also, it's a struct, so the members don't need m_ prefixes.

* page/DragController.cpp: Removed unneeded include of Node.h since it's included by
Element.h, which is also included.
(WebCore::DragController::draggableElement): Renamed from Node to Element and changed
the types and names of arguments accordingly. Also made this function handle a
startElement of 0 so callers don't need to. Also updated for changes to DragState members.
(WebCore::DragController::startDrag): Updated for changes to DragState members.
Since dragSource can only be an element, was able to get rid of isElementNode checks.

* page/DragController.h: Fixed style of forward declarations of structs. Updated for
change of draggableNode to draggableElement. Also removed declarations of nonexistent
selectionDraggingRect and doDrag functions.

* page/DragState.h: Rewrote practically this whole header. Added an include of Element
instead of Node since that's what we use now. Removed includes that are redundant.
There's no problem copying a DragState, so removed WTF_MAKE_NONCOPYABLE. There's no need
to allocate a DragState on the heap, so removed WTF_MAKE_FAST_ALLOCATED. The event dispatch
boolean is never set to a constant, so there's no need for a policy enum; it can just be
a boolean. Removed the "m_" prefixes from the struct members, since this is a struct with
public members and we don't use the prefix in those cases. Removed the word "drag" from the
struct member names since this entire struct is about dragging and has drag in its name.
Left the comments mostly intact, even though I'm not certain of their value.

* page/EventHandler.cpp:
(WebCore::EventHandler::handleMousePressEvent): Updated for changes to DragState.
(WebCore::EventHandler::eventMayStartDrag):
Use innerElement instead of innerNode to call draggableElement instead of draggableNode.
(WebCore::EventHandler::updateDragAndDrop): Updated for changes to DragState.
(WebCore::EventHandler::cancelDragAndDrop): Ditto.
(WebCore::EventHandler::handleWheelEvent): Added FIXME.
(WebCore::EventHandler::dragHysteresisExceeded): Updated for changes to DragState.
(WebCore::EventHandler::freeClipboard): Updated for changes to DragState. Also re-added
code to release the clipboard object, which is needed here to avoid keeping it around in
memory until the next drag.
(WebCore::EventHandler::dragSourceEndedAt): Updated for changes to DragState.
(WebCore::EventHandler::updateDragStateAfterEditDragIfNeeded): Ditto.
(WebCore::EventHandler::dispatchDragSrcEvent): Ditto.
(WebCore::EventHandler::handleDrag): Updated for changes to DragState. Use innerElement
instead of innerNode to call draggableElement instead of draggableNode. No longer need to
null check innerElement because draggableElement does that. Removed unneeded else that was
setting m_dragSrc to zero since it's guaranteed to already be zero.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150354 268f45cc-cd09-0410-ab3c-d52691b4dbfc