Document::topDocument() should return a reference

Document::topDocument() should return a reference
https://bugs.webkit.org/show_bug.cgi?id=127786

Reviewed by Darin Adler.

* accessibility/AccessibilityObject.cpp:
(WebCore::AccessibilityObject::mainFrame):
(WebCore::AccessibilityObject::topDocument):
* accessibility/ios/WebAccessibilityObjectWrapperIOS.mm:
(-[WebAccessibilityObjectWrapper accessibilityContainer]):
* dom/Document.cpp:
(WebCore::Document::~Document):
(WebCore::Document::createRenderTree):
(WebCore::Document::destroyRenderTree):
(WebCore::Document::clearAXObjectCache):
(WebCore::Document::existingAXObjectCache):
(WebCore::Document::axObjectCache):
(WebCore::Document::implicitClose):
(WebCore::Document::topDocument):
(WebCore::Document::topOrigin):
(WebCore::Document::webkitCancelFullScreen):
(WebCore::Document::webkitDidExitFullScreenForElement):
* dom/Document.h:
* page/DOMWindow.cpp:
(WebCore::DOMWindow::incrementScrollEventListenersCount):
(WebCore::DOMWindow::decrementScrollEventListenersCount):
* rendering/RenderEmbeddedObject.cpp:
(WebCore::RenderEmbeddedObject::isReplacementObscured):
* rendering/RenderView.cpp:
(WebCore::RenderView::RepaintRegionAccumulator::RepaintRegionAccumulator):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@162962 268f45cc-cd09-0410-ab3c-d52691b4dbfc