When computing download item icon, check if this.data is undefined

computeIcon_ can be called while this.data is still undefined, leading
to TypeErrors when interacting with the chrome://downloads page.

Bug: 917190
Change-Id: I4ef3fda1d53f3d9fbd9c80270594c6115b757bf4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1635434
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665969}

chrome/browser/resources/downloads/item.js

@@ -244,6 +244,7 @@ cr.define('downloads', function() {
      */
     computeIcon_: function() {
       if (loadTimeData.getBoolean('requestsApVerdicts') &&
+          this.data &&
           this.data.dangerType == downloads.DangerType.UNCOMMON_CONTENT) {
         return 'cr:error';
       }