Move PNM_SETSCROLLPOS to custom method

This CL splits PNM_SETSCROLLPOS out of OnNotify and places into a
method which is called as needed.

Change-Id: Ic144bc30bca57869f0679bef40e07b6a130e944c
Reviewed-on: https://pdfium-review.googlesource.com/7252
Reviewed-by: Nicolás Peña <npm@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>
diff --git a/fpdfsdk/pdfwindow/cpwl_wnd.h b/fpdfsdk/pdfwindow/cpwl_wnd.h
index 9ffae16..c9c4130 100644
--- a/fpdfsdk/pdfwindow/cpwl_wnd.h
+++ b/fpdfsdk/pdfwindow/cpwl_wnd.h
@@ -74,7 +74,6 @@
 #define PRES_TEXTOVERFLOW 0x0400L
 
 // notification messages
-#define PNM_SETSCROLLPOS 3
 #define PNM_SCROLLWINDOW 4
 #define PNM_LBUTTONDOWN 5
 #define PNM_LBUTTONUP 6
@@ -221,6 +220,7 @@
                         intptr_t wParam = 0,
                         intptr_t lParam = 0);
   virtual void SetScrollInfo(const PWL_SCROLL_INFO& info);
+  virtual void SetScrollPosition(float pos);
   virtual void SetFocus();
   virtual void KillFocus();
   virtual void SetCursor();