[bindings] Make ScriptValue::v8ValueFor(...) a 'const' method.

Source/bindings/core/v8/ScriptValue.h

Index: Source/bindings/core/v8/ScriptValue.h
diff --git a/Source/bindings/core/v8/ScriptValue.h b/Source/bindings/core/v8/ScriptValue.h
index d527228a69aea0d9434c44f6cc9f22b9bc1cbe54..2dc8b8892604edbc6af2ed29af52cebac572af93 100644
--- a/Source/bindings/core/v8/ScriptValue.h
+++ b/Source/bindings/core/v8/ScriptValue.h
@@ -172,7 +172,7 @@ public:
     // Returns v8Value() if a given ScriptState is the same as the
     // ScriptState which is associated with this ScriptValue. Otherwise
     // this "clones" the v8 value and returns it.
-    v8::Local<v8::Value> v8ValueFor(ScriptState*);
+    v8::Local<v8::Value> v8ValueFor(ScriptState*) const;
 
     bool toString(String&) const;