PDF Viewer: fix flakey test.

Suspected reason for flakiness is that the timing of cr-input shifting focus onto the
internal input sets it as the activeElement too late.

Bug: 855637
Change-Id: Ifdc3fa26c357ca6aa1e92180ec309cca4e455e07
Reviewed-on: https://chromium-review.googlesource.com/1112350
Commit-Queue: Scott Chen <scottchen@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569726}

chrome/test/data/pdf/basic_test.js

@@ -41,7 +41,7 @@ var tests = [
   function testIgnoreKeyEvents() {
     // Test that the traversal through the shadow DOM works correctly.
     var toolbar = document.getElementById('toolbar');
-    toolbar.$.pageselector.pageSelector.focus();
+    toolbar.$.pageselector.pageSelector.inputElement.focus();
     chrome.test.assertTrue(shouldIgnoreKeyEvents(toolbar));
 
     // Test case where the active element has a shadow root of its own.