Allows the PDF engine return the page index it is scrolling to

pdf/pdfium/pdfium_engine.h

Index: pdf/pdfium/pdfium_engine.h
diff --git a/pdf/pdfium/pdfium_engine.h b/pdf/pdfium/pdfium_engine.h
index e6392b4a482d433e56dbab35b45a9b770f5c6f8e..ed73bef9282a0aee8b558fe33b0791e22e6916a7 100644
--- a/pdf/pdfium/pdfium_engine.h
+++ b/pdf/pdfium/pdfium_engine.h
@@ -15,6 +15,7 @@
 #include <vector>
 
 #include "base/macros.h"
+#include "base/optional.h"
 #include "base/time/time.h"
 #include "pdf/document_loader.h"
 #include "pdf/pdf_engine.h"
@@ -248,6 +249,11 @@ class PDFiumEngine : public PDFEngine,
   // must have been called first.
   bool IsPageVisible(int index) const;
 
+  // Internal interface that caches the page index requested by the PDFium
+  // engine
+  // to be used during the interval the client has not finished handling it.
+  void ScrollToPage(int page);
+
   // Checks if a page is now available, and if so marks it as such and returns
   // true.  Otherwise, it will return false and will add the index to the given
   // array if it's not already there.
@@ -677,6 +683,10 @@ class PDFiumEngine : public PDFEngine,
   // CalculateVisiblePages()
   int most_visible_page_;
 
+  // Holds the page index requested by PDFium while there are in-flight
+  // messages being sent by the Browser <-> PDFium engine.
+  base::Optional<int> in_flight_visible_page_;
+
   // Set to true after FORM_DoDocumentJSAction/FORM_DoDocumentOpenAction have
   // been called. Only after that can we call FORM_DoPageAAction.
   bool called_do_document_action_;