PDF Viewer update: Fix positioning of the loading bar.

It regressed at r791545, as a result of other layout changes.
Should appear at the bottom of the toolbar, and not at the
bottom of the entire viewport.

Fixed: 1128040
Change-Id: Ia78cc531c62baa6b5a85c1fdd6abe19156d3106c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2410957
Auto-Submit: dpapad <dpapad@chromium.org>
Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Commit-Queue: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806804}

chrome/browser/resources/pdf/elements/viewer-pdf-toolbar-new.html

@@ -5,6 +5,7 @@
         0 4px 8px rgba(0, 0, 0, 0.06),
         0 1px 2px rgba(0, 0, 0, 0.3),
         0 2px 6px rgba(0, 0, 0, 0.15);
+    position: relative;
   }
 
   #toolbar {