Remove last CJX_ classes that do not meaningfully extend superclasses

Add a comment to cjx_object.h to describe new situation.

Change-Id: I8e5ae815d3f9cad8b022e83dc491a8c0ce07a62a
Reviewed-on: https://pdfium-review.googlesource.com/c/48950
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>

xfa/fxfa/parser/cxfa_sourceset.cpp

diff --git a/xfa/fxfa/parser/cxfa_sourceset.cpp b/xfa/fxfa/parser/cxfa_sourceset.cpp
index a968e4e..91be852 100644
--- a/xfa/fxfa/parser/cxfa_sourceset.cpp
+++ b/xfa/fxfa/parser/cxfa_sourceset.cpp
@@ -6,7 +6,7 @@
 
 #include "xfa/fxfa/parser/cxfa_sourceset.h"
 
-#include "fxjs/xfa/cjx_sourceset.h"
+#include "fxjs/xfa/cjx_model.h"
 #include "third_party/base/ptr_util.h"
 
 namespace {
@@ -28,6 +28,6 @@
                 XFA_Element::SourceSet,
                 nullptr,
                 kSourceSetAttributeData,
-                pdfium::MakeUnique<CJX_SourceSet>(this)) {}
+                pdfium::MakeUnique<CJX_Model>(this)) {}
 
 CXFA_SourceSet::~CXFA_SourceSet() = default;