Sort includes in xfa/.

This CL sorts all the includes in the xfa/ directory. There are a lot of IWYU
errors that have been fixed to make things compile again.

BUG=pdfium:65
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1722803002 .
diff --git a/xfa/include/fwl/theme/comboboxtp.h b/xfa/include/fwl/theme/comboboxtp.h
index 8b6a221..0cf13a4 100644
--- a/xfa/include/fwl/theme/comboboxtp.h
+++ b/xfa/include/fwl/theme/comboboxtp.h
@@ -7,8 +7,8 @@
 #ifndef XFA_INCLUDE_FWL_THEME_COMBOBOXTP_H_
 #define XFA_INCLUDE_FWL_THEME_COMBOBOXTP_H_
 
-class CFWL_WidgetTP;
-class CFWL_ComboBoxTP;
+#include "xfa/include/fwl/theme/widgettp.h"
+
 class CFWL_ComboBoxTP : public CFWL_WidgetTP {
  public:
   CFWL_ComboBoxTP();