Introduce isHTMLLabelElement and toHTMLLabelElement

Source/core/accessibility/AXObjectCache.cpp

Index: Source/core/accessibility/AXObjectCache.cpp
diff --git a/Source/core/accessibility/AXObjectCache.cpp b/Source/core/accessibility/AXObjectCache.cpp
index dfec8eb35fc1ef3983ce56deb5090c5b1993846f..3debe18072b80d7dc13c00e1c4e3cb68cc9e020b 100644
--- a/Source/core/accessibility/AXObjectCache.cpp
+++ b/Source/core/accessibility/AXObjectCache.cpp
@@ -791,7 +791,7 @@ void AXObjectCache::handleAttributeChanged(const QualifiedName& attrName, Elemen
         handleAriaRoleChanged(element);
     else if (attrName == altAttr || attrName == titleAttr)
         textChanged(element);
-    else if (attrName == forAttr && element->hasTagName(labelTag))
+    else if (attrName == forAttr && isHTMLLabelElement(element))
         labelChanged(element);
 
     if (!attrName.localName().string().startsWith("aria-"))
@@ -819,9 +819,7 @@ void AXObjectCache::handleAttributeChanged(const QualifiedName& attrName, Elemen
 
 void AXObjectCache::labelChanged(Element* element)
 {
-    ASSERT(element->hasTagName(labelTag));
-    HTMLElement* correspondingControl = static_cast<HTMLLabelElement*>(element)->control();
-    textChanged(correspondingControl);
+    textChanged(toHTMLLabelElement(element)->control());
 }
 
 void AXObjectCache::recomputeIsIgnored(RenderObject* renderer)