Fix plural message that couldn't be extracted

components/pdf_strings.grdp

 <?xml version="1.0" encoding="utf-8"?>
 <grit-part>
   <if expr="enable_plugins">
     <message name="IDS_PDF_NEED_PASSWORD" desc="A message asking the user for a password to open a PDF file.">
       This document is password protected.  Please enter a password.
     </message>
     <message name="IDS_PDF_PASSWORD_SUBMIT" desc="Button label for the buton to submit the PDF password form">
       Submit
     </message>
     <message name="IDS_PDF_PASSWORD_INVALID" desc="Error message when an invalid password is entered">
@@ skipping 33 matching lines @@
       Zoom in
     </message>
     <message name="IDS_PDF_TOOLTIP_ZOOM_OUT" desc="Button tooltip for the button which zooms out a PDF, so that the document appears smaller">
       Zoom out
     </message>
     <message name="IDS_PDF_LABEL_PAGE_NUMBER" desc="Label for the input field containing the current page number">
       Page number
     </message>
     <message name="IDS_PDF_PAGE_INDEX" desc="Accessible label for each page, describing the page number">
       {COUNT, plural,
+         =1 {Page 1}
          other {Page #}}
     </message>
   </if>
 </grit-part>