OOP PDF: Hide the loading indicator until the plugin loads.

Previously if the PDF plugin is disabled or click-to-play, the loading
indicator appeared over the click-to-play overlay. This hides the
loading indicator until a message is received from the plugin.

BUG=441132

Review URL: https://codereview.chromium.org/796223002

Cr-Commit-Position: refs/heads/master@{#308303}

chrome/browser/resources/pdf/index.css

@@ -19,6 +19,7 @@ viewer-page-indicator {
 }
 
 viewer-progress-bar {
+  visibility: hidden;
   z-index: 3;
 }
 

chrome/browser/resources/pdf/pdf.js

@@ -402,6 +402,7 @@ PDFViewer.prototype = {
       case 'setTranslatedStrings':
         this.passwordScreen_.text = message.data.getPasswordString;
         this.progressBar_.text = message.data.loadingString;
+        this.progressBar_.style.visibility = 'visible';
         this.errorScreen_.text = message.data.loadFailedString;
         break;
       case 'cancelStreamUrl':