Print Preview Componentization: Fix document title

Set the document title to the title of the tab being printed, so that
the task manager will show the correct task title.

Bug: 773928
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: If8f7fec12bd81cca71bdccf3fb9f040ac17bc3f0
Reviewed-on: https://chromium-review.googlesource.com/1104985Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568487}

chrome/browser/resources/print_preview/new/app.js

@@ -285,6 +285,9 @@ Polymer({
         this.recentDestinations_);
     this.isInAppKioskMode_ = settings.isInAppKioskMode;
     this.isInKioskAutoPrintMode_ = settings.isInKioskAutoPrintMode;
+
+    // This is only visible in the task manager.
+    document.head.querySelector('title').textContent = settings.documentTitle;
   },
 
   /**