[pdf] Defer page unloading in JS callback.

pdf/pdfium/pdfium_engine.cc

Index: pdf/pdfium/pdfium_engine.cc
diff --git a/pdf/pdfium/pdfium_engine.cc b/pdf/pdfium/pdfium_engine.cc
index f3ae32c109488f52f10438a9ddf030c339d2c431..878ee20a7c325e6ac076a520d5db86af9c4b9ba4 100644
--- a/pdf/pdfium/pdfium_engine.cc
+++ b/pdf/pdfium/pdfium_engine.cc
@@ -2287,7 +2287,13 @@ int PDFiumEngine::GetMostVisiblePage() {
   if (in_flight_visible_page_)
     return *in_flight_visible_page_;
 
+  // We can call GetMostVisiblePage through a callback from PDFium. We have
+  // to defer the page deletion otherwise we could potentially delete the page
+  // that originated the calling JS request and destroy the objects that are
+  // currently being used.
+  defer_page_unload_ = true;
   CalculateVisiblePages();
+  defer_page_unload_ = false;
   return most_visible_page_;
 }