Fix flaky NPE in oobe.js.

telemetry/telemetry/internal/backends/chrome/oobe.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.
 
 from functools import partial
 import logging
 
 from telemetry.core import exceptions
 from telemetry.internal.browser import web_contents
 
@@ skipping 19 matching lines @@
     return None
 
   def _GaiaWebviewContext(self):
     webview_contexts = self.GetWebviewContexts()
     if webview_contexts:
       return webview_contexts[0]
     return None
 
   def _ExecuteOobeApi(self, api, *args):
     logging.info('Invoking %s' % api)
-    self.WaitForJavaScriptCondition("typeof Oobe == 'function'", timeout=120)
+    self.WaitForJavaScriptCondition(
+        "typeof Oobe == 'function' && Oobe.readyForTesting", timeout=120)
 
     if self.EvaluateJavaScript(
         "typeof {{ @api }} == 'undefined'", api=api):
       raise exceptions.LoginException('%s js api missing' % api)
 
     # Example values:
     #   |api|:    'doLogin'
     #   |args|:   ['username', 'pass', True]
     #   Executes: 'doLogin("username", "pass", true)'
     self.ExecuteJavaScript('{{ @f }}({{ *args }})', f=api, args=args)
@@ skipping 65 matching lines @@
        document.getElementById({{ field }}).value= {{ value }};
        document.getElementById({{ next_field }}).click()""",
        field=field, value=value, next_field=next_field)
 
   def _WaitForField(self, field):
     gaia_webview_context = py_utils.WaitFor(self._GaiaWebviewContext, 5)
     py_utils.WaitFor(gaia_webview_context.HasReachedQuiescence, 20)
     gaia_webview_context.WaitForJavaScriptCondition(
         "document.getElementById({{ field }}) != null",
         field=field, timeout=20)