Hide CPDF_FormControl from CFFL_Button

The FFL side of the house manages PWL, and the CPDFSDK side of the
house manages CPDF, and so let's begin disentangling these.

-- Add a missing const while at it.

Change-Id: I0b8485cce44cf31d80e83107ceb72899ab12b958
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/100051
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/fpdfsdk/cpdfsdk_widget.h b/fpdfsdk/cpdfsdk_widget.h
index c7dad0b..68fad87 100644
--- a/fpdfsdk/cpdfsdk_widget.h
+++ b/fpdfsdk/cpdfsdk_widget.h
@@ -134,7 +134,8 @@
   uint32_t GetAppearanceAge() const { return m_nAppearanceAge; }
   uint32_t GetValueAge() const { return m_nValueAge; }
 
-  bool IsWidgetAppearanceValid(CPDF_Annot::AppearanceMode mode);
+  bool IsWidgetAppearanceValid(CPDF_Annot::AppearanceMode mode) const;
+  bool IsPushHighlighted() const;
   CFX_Matrix GetMatrix() const;
   CFX_FloatRect GetClientRect() const;
   CFX_FloatRect GetRotatedRect() const;