Add private PPAPI interfaces for PDFium accessibility.

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 ec3c80dcce9376ea7b79441ede33ba8995a2b355..e37ab3b80233d06a990cd0a98cdfe0145b3fd0a1 100644
--- a/pdf/pdfium/pdfium_engine.h
+++ b/pdf/pdfium/pdfium_engine.h
@@ -90,6 +90,7 @@ class PDFiumEngine : public PDFEngine,
   void SetGrayscale(bool grayscale) override;
   void OnCallback(int id) override;
   std::string GetPageAsJSON(int index) override;
+  void EnableAccessibility() override;
   bool GetPrintScaling() override;
   int GetCopiesToPrint() override;
   int GetDuplexType() override;