Add a helper for implementing Promise-valued properties.

Source/bindings/core/v8/ScriptPromise.h

Index: Source/bindings/core/v8/ScriptPromise.h
diff --git a/Source/bindings/core/v8/ScriptPromise.h b/Source/bindings/core/v8/ScriptPromise.h
index c58e6f9afb5916bff38104eb9795f3ea66d95af8..46df6daa933f883859cab797c4a0157c5e68abbd 100644
--- a/Source/bindings/core/v8/ScriptPromise.h
+++ b/Source/bindings/core/v8/ScriptPromise.h
@@ -93,6 +93,16 @@ public:
         m_promise.clear();
     }
 
+    bool operator==(const ScriptPromise& value) const
+    {
+        return m_promise == value.m_promise;
+    }
+
+    bool operator!=(const ScriptPromise& value) const
+    {
+        return !operator==(value);
+    }
+
     // Constructs and returns a ScriptPromise from |value|.
     // if |value| is not a Promise object, returns a Promise object
     // resolved with |value|.