Use public interface for PDF page view access

This becomes necessary as a result of the change introduced by
mozilla/pdf.js@b1c4b85d4fb7faba2b454d02d279644244bf282b

This method has existed for a long time so hopefully this won't
cause breakage for anyone on a slightly older FF.

h/static/scripts/annotator/anchoring/pdf.coffee

@@ -24,7 +24,7 @@ getNodeTextLayer = (node) ->
 
 
 getPage = (pageIndex) ->
-  return PDFViewerApplication.pdfViewer.pages[pageIndex]
+  return PDFViewerApplication.pdfViewer.getPageView(pageIndex)
 
 
 getPageTextContent = (pageIndex) ->

h/static/scripts/annotator/plugin/pdf.coffee

@@ -66,7 +66,10 @@ class PDF extends Annotator.Plugin
     refreshAnnotations = []
 
     # Check all the pages with text layers that have finished rendering.
-    for page in pdfViewer.pages when page.textLayer?.renderingDone
+    for pageIndex in [0...pdfViewer.pagesCount]
+      page = pdfViewer.getPageView(pageIndex)
+      continue unless page.textLayer?.renderingDone
+
       div = page.div ? page.el
       placeholder = div.getElementsByClassName('annotator-placeholder')[0]