[MD History] Factor out a common HistoryListBehavior.

ui/webui/resources/js/util.js

Index: ui/webui/resources/js/util.js
diff --git a/ui/webui/resources/js/util.js b/ui/webui/resources/js/util.js
index 293205804389a21425c26bc0c26756533d9e42ea..6caffd4b4027ba4965e28ead71e04e905545499b 100644
--- a/ui/webui/resources/js/util.js
+++ b/ui/webui/resources/js/util.js
@@ -373,6 +373,30 @@ function quoteString(str) {
   return str.replace(/([\\\.\+\*\?\[\^\]\$\(\)\{\}\=\!\<\>\|\:])/g, '\\$1');
 }
 
+/**
+ * Returns a promise which is resolved when |eventName| is fired on |element|
+ * and |predicate| is true.
+ * @param {HTMLElement} element
+ * @param {string} eventName
+ * @param {function(Event): boolean} predicate

[tsergeant, 2016/08/05 01:39:09]

@return {Promise}

[calamity, 2016/08/09 02:56:00]

Done.

+ */
+function waitForEvent(element, eventName, predicate) {

[tsergeant, 2016/08/05 01:39:09]

I can conceive of situations where this would be useful in real code... but
considering at the moment this is only used in a test, I don't think there's any
need to ship this out with every WebUI.

I think we should just put this in test_util for now.

[calamity, 2016/08/09 02:56:00]

Done.

+  if (!predicate)
+    predicate = function() { return true; };
+
+  return new Promise(function(resolve) {
+    var listener = function(e) {
+      if (!predicate(e))
+        return;
+
+      resolve();
+      element.removeEventListener(eventName, listener);
+    }
+
+    element.addEventListener(eventName, listener);
+  });
+}
+
 // <if expr="is_ios">
 // Polyfill 'key' in KeyboardEvent for iOS.
 // This function is not intended to be complete but should