Create classes for XFA JS objects

This CL creates classes for the various XFA JS objects. The method
calls from xfa_basic_data_element_script have been modified to call
these methods instead of methods in CJX_Object.

Change-Id: Idb2de38656f7e8edc559d44921eba25073e6c806
Reviewed-on: https://pdfium-review.googlesource.com/21110
Commit-Queue: dsinclair <dsinclair@chromium.org>
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
diff --git a/fxjs/xfa/cjx_model.h b/fxjs/xfa/cjx_model.h
index 17b8d3d..fae1cc7 100644
--- a/fxjs/xfa/cjx_model.h
+++ b/fxjs/xfa/cjx_model.h
@@ -21,6 +21,9 @@
   JS_METHOD(createNode, CJX_Model);
   JS_METHOD(isCompatibleNS, CJX_Model);
 
+  JS_PROP(aliasNode);
+  JS_PROP(context);
+
  private:
   static const CJX_MethodSpec MethodSpecs[];
 };