Cleanup IOn* methods and CPWL_CBEdit

This CL removes various IOn* methods and consolidates code where
possible. The CPWL_CBEdit class is also removed as it was a subclass
of CPWL_Edit which added no extra functionality.

Change-Id: Ifac579ab252b7ca64f8c9b3caeac443683a39c58
Reviewed-on: https://pdfium-review.googlesource.com/7279
Commit-Queue: dsinclair <dsinclair@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/fpdfsdk/pdfwindow/cpwl_combo_box.cpp b/fpdfsdk/pdfwindow/cpwl_combo_box.cpp
index ace59a8..660f8cd 100644
--- a/fpdfsdk/pdfwindow/cpwl_combo_box.cpp
+++ b/fpdfsdk/pdfwindow/cpwl_combo_box.cpp
@@ -278,7 +278,7 @@
   if (m_pEdit)
     return;
 
-  m_pEdit = new CPWL_CBEdit();
+  m_pEdit = new CPWL_Edit();
   m_pEdit->AttachFFLData(m_pFormFiller.Get());
 
   PWL_CREATEPARAM ecp = cp;