Rework "Make common page base class."

Re-landing of https://pdfium-review.googlesource.com/c/pdfium/+/32892

This time, however, we do not build on the previous CL which cached
pages. This CL by itself should be OK but was reverted only because
it was blocking earlier reverts.

Change-Id: I067d5f07373eeac6cced5d0c113ea40e5f8dcd15
Reviewed-on: https://pdfium-review.googlesource.com/34910
Commit-Queue: dsinclair <dsinclair@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
diff --git a/fpdfsdk/cpdfsdk_interform.cpp b/fpdfsdk/cpdfsdk_interform.cpp
index 1d8002a..1e24746 100644
--- a/fpdfsdk/cpdfsdk_interform.cpp
+++ b/fpdfsdk/cpdfsdk_interform.cpp
@@ -358,7 +358,7 @@
     if (!pWidget)
       continue;
 
-    UnderlyingPageType* pPage = pWidget->GetUnderlyingPage();
+    IPDF_Page* pPage = pWidget->GetPage();
     FX_RECT rect = formfiller->GetViewBBox(
         m_pFormFillEnv->GetPageView(pPage, false), pWidget);
     m_pFormFillEnv->Invalidate(pPage, rect);