[I-Spy] Add support for rebaselining expectations from the web UI.

chrome/test/functional/ispy/client/dom.py

+# Copyright 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+
+def GetScriptToWaitForUnchangingDOM():
+  """Gets Javascript that waits until the DOM is stable for 5 seconds.
+
+  Times out if the DOM is not stable within 30 seconds.
+
+  Returns:
+    Javascript as a string.
+  """
+  return """
+    var target = document.body;
+    var callback = arguments[arguments.length - 1]
+
+    var timeout_id = setTimeout(function() {
+      callback()
+    }, 5000);
+
+    var observer = new MutationObserver(function(mutations) {
+      clearTimeout(timeout_id);
+      timeout_id = setTimeout(function() {
+        callback();
+      }, 5000);
+    }).observe(target, {attributes: true, childList: true,
+      characterData: true, subtree: true});
+  """