Add common base class between CXFA_FFComboBox and CXFA_FFListBox

This CL adds a CXFA_FFDropDown to serve as a base class for the
CXFA_FFComboBox and CXFA_FFListBox and adds a virtual InsertItem and
DeleteItem methods.

Change-Id: I325ffc579ed42a4755bae0c4d18667f8a9458950
Reviewed-on: https://pdfium-review.googlesource.com/24550
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 4fe979e..bcb0e7e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -2023,6 +2023,8 @@
       "xfa/fxfa/cxfa_ffdoc.h",
       "xfa/fxfa/cxfa_ffdocview.cpp",
       "xfa/fxfa/cxfa_ffdocview.h",
+      "xfa/fxfa/cxfa_ffdropdown.cpp",
+      "xfa/fxfa/cxfa_ffdropdown.h",
       "xfa/fxfa/cxfa_ffexclgroup.cpp",
       "xfa/fxfa/cxfa_ffexclgroup.h",
       "xfa/fxfa/cxfa_fffield.cpp",