[pdf] Reset defer_page_unload to original value.

In the PdfiumEngine::GetMostVisiblePage() we force the system to defer page
unloading because it can be called during a callback. That callback can be
triggered from a ::HandleEvent() call. Currently we will always reset the
defer_page_unload_ to false in ::GetMostVisiblePage().

This Cl updates the code to reset defer_page_unload_ to the value it contained
prior to the call to ::GetMostVisiblePage().

Review-Url: https://codereview.chromium.org/2554563002
Cr-Commit-Position: refs/heads/master@{#436590}

pdf/pdfium/pdfium_engine.cc

@@ -12,6 +12,7 @@
 #include <memory>
 #include <set>
 
+#include "base/auto_reset.h"
 #include "base/i18n/encoding_detection.h"
 #include "base/i18n/icu_string_conversions.h"
 #include "base/lazy_instance.h"
@@ -2470,9 +2471,8 @@ int PDFiumEngine::GetMostVisiblePage() {
   // 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;
+  base::AutoReset<bool> defer_page_unload_guard(&defer_page_unload_, true);
   CalculateVisiblePages();
-  defer_page_unload_ = false;
   return most_visible_page_;
 }