Tidy up JS_Value.h

Use ToV8Object() instead of CJS_Value cast operator.
Add some missing consts / explicits.
Move code into empty namespace.

Review-Url: https://codereview.chromium.org/2172813002
diff --git a/fpdfsdk/javascript/JS_Define.h b/fpdfsdk/javascript/JS_Define.h
index 6aa1dec..e120758 100644
--- a/fpdfsdk/javascript/JS_Define.h
+++ b/fpdfsdk/javascript/JS_Define.h
@@ -92,7 +92,7 @@
                                             sError));
     return;
   }
-  info.GetReturnValue().Set((v8::Local<v8::Value>)value);
+  info.GetReturnValue().Set(value.ToV8Value());
 }
 
 template <class C,
@@ -383,7 +383,7 @@
     FXJS_Error(isolate, JSFormatErrorString(class_name, "GetProperty", sError));
     return;
   }
-  info.GetReturnValue().Set((v8::Local<v8::Value>)value);
+  info.GetReturnValue().Set(value.ToV8Value());
 }
 
 template <class Alt>