PDF Viewer Update: Scroll thumbnail into view

When the active page changes, scroll the thumbnail into the visible area
of the thumbnail bar.

Bug: 652400
Change-Id: I6333a66b88202b7dabcb1a0f287b3fbee2d3b701
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404564
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806392}

chrome/browser/resources/pdf/elements/viewer-thumbnail.js

@@ -19,6 +19,7 @@ export class ViewerThumbnailElement extends PolymerElement {
     return {
       isActive: {
         type: Boolean,
+        observer: 'isActiveChanged_',
         reflectToAttribute: true,
       },
 
@@ -26,6 +27,13 @@ export class ViewerThumbnailElement extends PolymerElement {
     };
   }
 
+  /** @private */
+  isActiveChanged_() {
+    if (this.isActive) {
+      this.scrollIntoView({block: 'nearest'});
+    }
+  }
+
   /** @private */
   onClick_() {
     this.dispatchEvent(new CustomEvent('change-page', {