Generate FrameTree::uniqueName with a URL attribute value.

Source/core/page/FrameTree.h

Index: Source/core/page/FrameTree.h
diff --git a/Source/core/page/FrameTree.h b/Source/core/page/FrameTree.h
index 94fc05b9df71546a45d9e743db49b482e7ad270c..f694dd516e168f5bf7a58e0aab45b73e8123d420 100644
--- a/Source/core/page/FrameTree.h
+++ b/Source/core/page/FrameTree.h
@@ -35,7 +35,9 @@ namespace WebCore {
 
         const AtomicString& name() const { return m_name; }
         const AtomicString& uniqueName() const { return m_uniqueName; }
-        void setName(const AtomicString&);
+        // If |name| is not empty, |fallbackName| is ignored. Otherwise,
+        // |fallbackName| is used as a source of uniqueName.
+        void setName(const AtomicString& name, const AtomicString& fallbackName = nullAtom);
 
         LocalFrame* parent() const;
         LocalFrame* top() const;