REGRESSION (r52008): Middle-clicking on a linked image starts a pan scroll,
but should follow the link instead.

Reviewed by Adam Roben.

WebCore: 

The node itself isn't the only possible node that can be a link (that was the
original check), any of its ancestors could be links as well, we need to climb
up the tree to see. Created a new function (enclosingAnchorNode), that finds a
node's enclosing anchor element (if it exists), that we can share between RenderLayer
and Node, and also added tests that test pan scrolling behavior in image maps.

Tests: platform/win/fast/events/panScroll-image-no-scroll.html
       platform/win/fast/events/panScroll-imageMap-href-no-scroll.html
       platform/win/fast/events/panScroll-imageMap-noHref-scroll.html

* dom/Node.cpp:
(WebCore::Node::enclosingAnchorNode):
(WebCore::Node::defaultEventHandler):
* dom/Node.h:
* rendering/RenderLayer.cpp:
(WebCore::RenderLayer::hitTest):

LayoutTests: 

Added tests that test pan scrolling behavior when you middle click on images with
links, and if you click on parts on image maps that are links, and that aren't.

* platform/win/fast/events/panScroll-image-no-scroll-expected.txt: Added.
* platform/win/fast/events/panScroll-image-no-scroll.html: Added.
* platform/win/fast/events/panScroll-imageMap-href-no-scroll-expected.txt: Added.
* platform/win/fast/events/panScroll-imageMap-href-no-scroll.html: Added.
* platform/win/fast/events/panScroll-imageMap-noHref-scroll-expected.txt: Added.
* platform/win/fast/events/panScroll-imageMap-noHref-scroll.html: Added.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@52341 268f45cc-cd09-0410-ab3c-d52691b4dbfc