Add CefPrintHandler::OnPrintStart callback (issue #1736)

libcef/browser/printing/print_dialog_linux.h

@@ -37,6 +37,10 @@ class CefPrintDialogLinux
   static gfx::Size GetPdfPaperSize(
     printing::PrintingContextLinux* context);
 
+  // Notify the client when printing has started.
+  static void OnPrintStart(int render_process_id,
+                           int render_routing_id);
+
   // printing::CefPrintDialogLinuxInterface implementation.
   void UseDefaultSettings() override;
   bool UpdateSettings(printing::PrintSettings* settings) override;