Remove FWL_WGTHITTEST_* defines in favour of enum class.

This Cl updates the FWL_WGTHITTEST_ defines to an enum class and fixes up any
code issues.

Review-Url: https://codereview.chromium.org/1948583002
diff --git a/BUILD.gn b/BUILD.gn
index afedfd2..e67e17f 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1046,6 +1046,7 @@
       "xfa/fwl/core/ifwl_widget.h",
       "xfa/fwl/core/ifwl_widgetdelegate.h",
       "xfa/fwl/core/ifwl_widgetmgr.h",
+      "xfa/fwl/core/include/fwl_widgethit.h",
       "xfa/fwl/core/include/ifwl_adaptertimermgr.h",
       "xfa/fwl/lightwidget/cfwl_barcode.cpp",
       "xfa/fwl/lightwidget/cfwl_barcode.h",