WebUI: Add fallback for search engine favicons

This CL adds a default fallback of a favicon with an empty URL to
display the default globe icon, and also sends the search engine URL
as a fallback if the search engine favicon URL is not available.

Bug: 1002840
Change-Id: Ie1662e5b3c0e95418abc9bb4bf9a5306f15668d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1798982Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695850}

chrome/browser/resources/settings/search_engines_page/search_engine_entry.html

@@ -42,7 +42,10 @@
 
     <div class="list-item" focus-row-container>
       <div id="name-column">
-        <site-favicon favicon-url="[[engine.iconURL]]"></site-favicon>
+        <site-favicon
+            favicon-url="[[engine.iconURL]]"
+            url="[[engine.url]]">
+        </site-favicon>
         <div>[[engine.displayName]]</div>
       </div>
       <div id="keyword-column"><div>[[engine.keyword]]</div></div>

chrome/browser/resources/settings/site_favicon.js

@@ -17,7 +17,7 @@ Polymer({
 
   /** @private */
   getBackgroundImage_: function() {
-    let backgroundImage = 'none';
+    let backgroundImage = cr.icon.getFavicon('');
     if (this.faviconUrl) {
       const url = this.ensureUrlHasScheme_(this.faviconUrl);
       backgroundImage = cr.icon.getFavicon(url);