Replace static_casts with to* helper functions.

      https://bugs.webkit.org/show_bug.cgi?id=112296

Reviewed by Kentaro Hara.

to* helper functions are preferred over static_cast calls since they
help to catch bad casts easily on the testing infrastructure.

Source/WebCore:

* accessibility/AXObjectCache.cpp:
(WebCore::nodeHasRole):
* accessibility/AccessibilityObject.cpp:
(WebCore::AccessibilityObject::hasAttribute):
(WebCore::AccessibilityObject::getAttribute):
* accessibility/AccessibilityRenderObject.cpp:
(WebCore::AccessibilityRenderObject::anchorElement):
(WebCore::AccessibilityRenderObject::helpText):
(WebCore::AccessibilityRenderObject::checkboxOrRadioRect):
(WebCore::AccessibilityRenderObject::titleUIElement):
(WebCore::AccessibilityRenderObject::computeAccessibilityIsIgnored):
(WebCore::AccessibilityRenderObject::accessKey):
(WebCore::AccessibilityRenderObject::setElementAttributeValue):
(WebCore::AccessibilityRenderObject::setValue):
(WebCore::AccessibilityRenderObject::activeDescendant):
(WebCore::AccessibilityRenderObject::handleActiveDescendantChanged):
(WebCore::AccessibilityRenderObject::correspondingLabelForControlElement):
(WebCore::AccessibilityRenderObject::inheritsPresentationalRole):
(WebCore::AccessibilityRenderObject::setAccessibleName):
(WebCore::AccessibilityRenderObject::stringRoleForMSAA):
* bindings/gobject/WebKitDOMBinding.cpp:
(WebKit::createWrapper):
* bindings/js/JSClipboardCustom.cpp:
(WebCore::JSClipboard::setDragImage):
* bindings/js/JSElementCustom.cpp:
(WebCore::toJSNewlyCreated):
* bindings/js/JSNodeCustom.cpp:
(WebCore::createWrapperInline):
* bindings/v8/custom/V8ClipboardCustom.cpp:
(WebCore::V8Clipboard::setDragImageMethodCustom):
* bindings/v8/custom/V8ElementCustom.cpp:
(WebCore::wrap):
* bindings/v8/custom/V8NodeCustom.cpp:
(WebCore::wrap):
* dom/Document.cpp:
(WebCore::Document::importNode):
(WebCore::Document::recalcStyle):
(WebCore::Document::setFocusedNode):
(WebCore::Document::updateFocusAppearanceTimerFired):
* dom/Docu