cr-network-list-item: Only fire 'network-connected' on change

Currently 'network-connected' is fired much to often, causing
significant slowdowns on slower devices, especially in
chrome://network (debugging) but potentially to a lesser degree
in oobe and Settings.

Bug: none
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I171e2015b7257f9e19b383687ae20282c0eb6c4c
Reviewed-on: https://chromium-review.googlesource.com/747077Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513267}

ui/webui/resources/cr_elements/chromeos/network/cr_network_list_item.js

@@ -50,6 +50,12 @@ Polymer({
       reflectToAttribute: true,
       computed: 'getItemName_(item)',
     },
+
+    /**
+     * The cached ConnectionState for the network.
+     * @type {!CrOnc.ConnectionState|undefined}
+     */
+    connectionState_: String,
   },
 
   behaviors: [CrPolicyNetworkBehavior],
@@ -66,10 +72,14 @@ Polymer({
 
   /** @private */
   networkStateChanged_: function() {
-    if (this.networkState &&
-        this.networkState.ConnectionState == CrOnc.ConnectionState.CONNECTED) {
+    if (!this.networkState)
+      return;
+    var connectionState = this.networkState.ConnectionState;
+    if (connectionState == this.connectionState_)
+      return;
+    this.connectionState_ = connectionState;
+    if (connectionState == CrOnc.ConnectionState.CONNECTED)
       this.fire('network-connected', this.networkState);
-    }
   },
 
   /**