Shift-Click doesn't extend selection in internal PDF plugin

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 d7b48355666c4fddb257b85e22d19065e89b5ff9..9d6fd889ac3660adc40e1a9e1dec2b12da4d1331 100644
--- a/pdf/pdfium/pdfium_engine.h
+++ b/pdf/pdfium/pdfium_engine.h
@@ -423,6 +423,11 @@ class PDFiumEngine : public PDFEngine,
   // Common code shared by RotateClockwise() and RotateCounterclockwise().
   void RotateInternal();
 
+  // Helper function to handle mouse click when shift is already selected.
+  void HandleShiftClick(const int last_page_index,
+                        const int page_index,
+                        const int char_index);
+
   // FPDF_FORMFILLINFO callbacks.
   static void Form_Invalidate(FPDF_FORMFILLINFO* param,
                               FPDF_PAGE page,
@@ -709,6 +714,9 @@ class PDFiumEngine : public PDFEngine,
   // to false after the user finishes getting their password.
   bool getting_password_;
 
+  // keep value of char index at which last mouse down happened.
+  int last_char_index_;
+
   DISALLOW_COPY_AND_ASSIGN(PDFiumEngine);
 };