[CUPS] Fix the regression that "Add" button is disabled on manual-add-printer dialog.

It's a regression caused by https://codereview.chromium.org/2979903003/.
We need to make sure newPrinter is not a null object when automatically
switch from auto-discovery dialog to manual-add-printer dialog.

Bug: 744551
Change-Id: If535c64aa2bb716e3f8e813852b556f6f6f8f796
Reviewed-on: https://chromium-review.googlesource.com/575381Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487319}

chrome/browser/resources/settings/printing_page/cups_add_printer_dialog.js

@@ -537,6 +537,7 @@ Polymer({
     // "ADD PRINTER" button.
     if (!this.previousDialog_) {
       this.$$('add-printer-discovery-dialog').close();
+      this.newPrinter = getEmptyPrinter_();
       this.openManuallyAddPrinterDialog_();
     }
   },