Add toHTMLDialogElement(), and use it.

We'd like to avoid bare static_cast<>.

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23828005

git-svn-id: svn://svn.chromium.org/blink/trunk@157429 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/dom/Document.cpp

@@ -4982,7 +4982,7 @@ HTMLDialogElement* Document::activeModalDialog() const
 {
     if (m_topLayerElements.isEmpty())
         return 0;
-    return static_cast<HTMLDialogElement*>(m_topLayerElements.last().get());
+    return toHTMLDialogElement(m_topLayerElements.last().get());
 }
 
 void Document::webkitExitPointerLock()

third_party/WebKit/Source/core/html/HTMLDialogElement.h

@@ -61,6 +61,12 @@ private:
     String m_returnValue;
 };
 
+inline HTMLDialogElement* toHTMLDialogElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(HTMLNames::dialogTag));
+    return static_cast<HTMLDialogElement*>(node);
+}
+
 } // namespace WebCore
 
 #endif