Middle-mouse button not firing mousedown DOM event when autoscroll happens.
        
Reviewed by Adam Roben.

WebCore: 

This fix moves the pan scrolling code into Node.cpp, as part of the default event handler.
We get two wins out of this:
    1) Mousedown DOM events are now fired before pan scrolling.
    2) Calling event.preventDefault() on a middle mouse button will now prevent pan scrolling.
        
Added tests to show that middle mousedown DOM events are fired when panscrolling happens, and
that when event.preventDefault is called, pan scrolling doesn't happen.

Tests: platform/win/fast/events/panScroll-event-fired.html
       platform/win/fast/events/panScroll-preventDefault.html

* dom/Node.cpp:
(WebCore::Node::defaultEventHandler): Moved pan scrolling starting code to here.
* page/EventHandler.cpp:
(WebCore::EventHandler::startPanScrolling): Function to initialize scrolling variables and start timer.
(WebCore::EventHandler::handleMousePressEvent): From here.
* page/EventHandler.h:

LayoutTests: 

Added tests to show that middle mousedown DOM events are fired when panscrolling happens, and
that when event.preventDefault is called, pan scrolling doesn't happen.

* platform/win/fast/events/panScroll-event-fired-expected.txt: Added.
* platform/win/fast/events/panScroll-event-fired.html: Added.
* platform/win/fast/events/panScroll-preventDefault-expected.txt: Added.
* platform/win/fast/events/panScroll-preventDefault.html: Added.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@52008 268f45cc-cd09-0410-ab3c-d52691b4dbfc