Reviewed by Alexey Proskuryakov.

        svg/custom/use-instanceRoot-event-bubbling.xhtml is flakey
        https://bugs.webkit.org/show_bug.cgi?id=33835

        mouseEvent fires mutiple times in svg/custom/use-instanceRoot-as-event-target.xhtml
        https://bugs.webkit.org/show_bug.cgi?id=32519

        Test: svg/custom/use-instanceRoot-event-listener-liveness.xhtml

        Stabilize <use> scripting support - use tests are reliable now (tested using --repeach-each 50 --random -p)

        Do not reclone trees anymore because of event listener changes, instead keep correspondingElement & shadowTreeElement
        synchronized for each SVGElementInstance - any mutations on any event listeners are live, and take immediate effect,
        w/o having to rely on a reclone - this was the root of several race conditions making the <use> tests flakey.

        Fix SVGUseElement::instanceRoot() to force shadow tree creation, even if it was not attached so far - we can't wait
        for finishedParsing() to be called which would recalculate the document style and attach the shadow tree as result.
        This is now matching Operas behaviour.

        Optimize createAttributeEventListener() to not create event listeners if the supplied Attribute isNull() - otherwhise
        DOM calls like removeAttribute("onclick") cause a temporary JSEventListener to be created, added to the event listener
        cache and removed afterwards.

        * bindings/js/ScriptEventListener.cpp:
        (WebCore::createAttributeEventListener): Return early if the supplied attribute is null as discussed with Geoffrey.
        * bindings/v8/ScriptEventListener.cpp:
        (WebCore::createAttributeEventListener): Ditto.
        * dom/Node.cpp: Synchronize event listeners with all element instances, instead of marking the use elements to reclone.
        (WebCore::instancesForSVGElement):
        (WebCore::tryAddEventListener):
        (WebCore::Node::addEventListener): When adding a listener, get a list of element instances and add it their as well.
        (WebCore::tryRemoveEventListener):
        (WebCore::Node::removeEventListener): Ditto for removals, but with special logic for listeners created from markup (see comments)
        * svg/SVGElementInstance.cpp: Cleaned up, removing unncessary checks of correspondingElement() - there is an ASSERT in the ctor.
        (WebCore::SVGElementInstance::invalidateAllInstancesOfElement):
        (WebCore::SVGElementInstance::scriptExecutionContext):
        (WebCore::SVGElementInstance::addEventListener):
        (WebCore::SVGElementInstance::removeEventListener):
        (WebCore::SVGElementInstance::removeAllEventListeners):
        (WebCore::SVGElementInstance::eventTargetData):
        (WebCore::SVGElementInstance::ensureEventTargetData):
        * svg/SVGUseElement.cpp: Call document()->updateLayoutIgnorePendingStylesheets() to force shadow tree creation, just like CSSStyleDecl works.
        (WebCore::SVGUseElement::instanceRoot):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@53564 268f45cc-cd09-0410-ab3c-d52691b4dbfc