PDF Viewer: Restore viewer-toolbar-dropdown box shadow.

It was caused by a missing dependency.

Fixed: 1016699
Change-Id: I866a033782a53b2c6c1bb04479a058335cafe646
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1874189Reviewed-by: John Lee <johntlee@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708327}

chrome/browser/resources/pdf/elements/viewer-toolbar-dropdown.js

@@ -5,6 +5,7 @@
 import 'chrome://resources/cr_elements/cr_icon_button/cr_icon_button.m.js';
 import 'chrome://resources/cr_elements/icons.m.js';
 import 'chrome://resources/cr_elements/shared_vars_css.m.js';
+import 'chrome://resources/polymer/v3_0/paper-styles/shadow.js';
 
 import {html, Polymer} from 'chrome://resources/polymer/v3_0/polymer/polymer_bundled.min.js';