Remove PDFThumbnailViewer_ensureThumbnailVisible

Functionality wise, `ensureThumbnailVisible` is essentially just a shorthand for `scrollThumbnailIntoView`. (And note that `PDFViewer` doesn't implement a `ensurePageVisible` method.)

The only remaining usage of `PDFThumbnailViewer_ensureThumbnailVisible` is inside `PDFPresentationMode`, which introduces an otherwise unnecessary `PDFThumbnailViewer` dependency there.

We're already relying on the `presentationmodechanged` event, in various files, to track the state of Presentation Mode. Thus we can simply listen for that event in `PDFSidebar` too, and update the thumbnails if necessary.

web/pdf_sidebar.js

@@ -317,6 +317,14 @@ var PDFSidebar = (function PDFSidebarClosure() {
           self.switchView(SidebarView.THUMBS);
         }
       });
+
+      // Update the thumbnailViewer, if visible, when exiting presentation mode.
+      window.addEventListener('presentationmodechanged', function(evt) {
+        if (!evt.detail.active && !evt.detail.switchInProgress &&
+            self.isThumbnailViewVisible) {
+          self._updateThumbnailViewer();
+        }
+      });
     },
   };