Add const versions of some toHTMLFooElement().

Source/core/html/HTMLFrameOwnerElement.h

Index: Source/core/html/HTMLFrameOwnerElement.h
diff --git a/Source/core/html/HTMLFrameOwnerElement.h b/Source/core/html/HTMLFrameOwnerElement.h
index 9cbe4bb94b18ac414b9057da1a95f7c7e6fa95ea..ed1b7abbbd26b6ec7f25e979e8616d0ceea740f3 100644
--- a/Source/core/html/HTMLFrameOwnerElement.h
+++ b/Source/core/html/HTMLFrameOwnerElement.h
@@ -83,6 +83,12 @@ inline HTMLFrameOwnerElement* toHTMLFrameOwnerElement(Node* node)
     return static_cast<HTMLFrameOwnerElement*>(node);
 }
 
+inline const HTMLFrameOwnerElement* toHTMLFrameOwnerElement(const Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->isFrameOwnerElement());
+    return static_cast<const HTMLFrameOwnerElement*>(node);
+}
+
 class SubframeLoadingDisabler {
 public:
     explicit SubframeLoadingDisabler(Node* root)