MD Extensions: Drop unnecessary work when first populating the list.

During initial population of the extensions and apps lists, addItem()
was called N times, which internally performs a linear search N times,
as well as triggres splice() notifications that are unnecessary.

As revealed by DevTools (for a total of 30 extensions and apps):
 - before: the N calls to addItem() were taking ~22ms total
 - after: The initialization of the lists takes ~5ms total 

Bug: 764126
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: If66d752cd3e7ac4b4ecd01ea4964ca6fd8d8f0db
Reviewed-on: https://chromium-review.googlesource.com/746967Reviewed-by: Dave Schuyler <dschuyler@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512989}

chrome/browser/resources/md_extensions/manager.js

@@ -248,6 +248,24 @@ cr.define('extensions', function() {
           this.apps[this.getIndexInList_('apps', id)];
     },
 
+    /**
+     * Categorizes |items| to apps and extensions and initializes those lists.
+     * This is faster than calling |addItem| multiple times.
+     * @param {!Array<!chrome.developerPrivate.ExtensionInfo>} items
+     */
+    initAppsAndExtensions(items) {
+      items.sort(compareExtensions);
+      let apps = [];
+      let extensions = [];
+      for (let i of items) {
+        let list = this.getListId_(i) === 'apps' ? apps : extensions;
+        list.push(i);
+      }
+
+      this.apps = apps;
+      this.extensions = extensions;
+    },
+
     /**
      * Creates and adds a new extensions-item element to the list, inserting it
      * into its sorted position in the relevant section.

chrome/browser/resources/md_extensions/service.js

@@ -44,9 +44,7 @@ cr.define('extensions', function() {
       chrome.developerPrivate.getExtensionsInfo(
           {includeDisabled: true, includeTerminated: true}, extensions => {
             this.extensions_ = extensions;
-            for (let extension of extensions)
-              this.manager_.addItem(extension);
-
+            this.manager_.initAppsAndExtensions(extensions);
             this.manager_.initPage();
           });
       chrome.developerPrivate.getProfileConfiguration(