Preload Highlight.js on Diff Page

Change-Id: I391b56d02aaf87fa82a97cb0565f51dc725e6cf1

polygerrit-ui/app/elements/shared/gr-lib-loader/gr-lib-loader.js

@@ -21,6 +21,7 @@ import {LegacyElementMixin} from '@polymer/polymer/lib/legacy/legacy-element-mix
 import {PolymerElement} from '@polymer/polymer/polymer-element.js';
 import {htmlTemplate} from './gr-lib-loader_html.js';
 
+// preloaded in PolyGerritIndexHtml.soy
 const HLJS_PATH = 'bower_components/highlightjs/highlight.min.js';
 
 /** @extends PolymerElement */

resources/com/google/gerrit/httpd/raw/PolyGerritIndexHtml.soy

@@ -102,6 +102,7 @@
     {if $userIsAuthenticated}
       <link rel="preload" href="{$canonicalPath}/{$changeRequestsPath}/drafts" as="fetch" type="application/json" crossorigin="anonymous"/>{\n}
     {/if}
+    <link rel="preload" href="{$staticResourcePath}/bower_components/highlightjs/highlight.min.js" as="script"/>
   {/if}
 
   {if $useGoogleFonts}