Use enum for tracking form field types

Within PDFium use enum class for better type safety when working with
form field types. These values will still be converted to ints as part
of the public API, since that is the existing API.

This work is preperation for extending the number of form field types
to have more specific entries for XFA.

BUG=pdfium:952,chromium:763129,chromium:592758

Change-Id: Ie6c29f02ae22be782ff36eb87d27f1a4bf2c099e
Reviewed-on: https://pdfium-review.googlesource.com/22742
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
diff --git a/fpdfsdk/formfiller/cffl_formfiller.cpp b/fpdfsdk/formfiller/cffl_formfiller.cpp
index f86cb68..d3aa37e 100644
--- a/fpdfsdk/formfiller/cffl_formfiller.cpp
+++ b/fpdfsdk/formfiller/cffl_formfiller.cpp
@@ -268,9 +268,9 @@
 
   bool bDestroyPDFWindow;
   switch (m_pWidget->GetFieldType()) {
-    case FIELDTYPE_PUSHBUTTON:
-    case FIELDTYPE_CHECKBOX:
-    case FIELDTYPE_RADIOBUTTON:
+    case FormFieldType::kPushButton:
+    case FormFieldType::kCheckBox:
+    case FormFieldType::kRadioButton:
       bDestroyPDFWindow = true;
       break;
     default: