Add Incognito test case

BUG=

Review-Url: https://codereview.chromium.org/2610003003
Cr-Commit-Position: refs/heads/master@{#441488}

tools/chrome_proxy/webdriver/incognito.py

+# Copyright 2017 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.
+
+import common
+from common import TestDriver
+from common import IntegrationTest
+
+
+class Incognito(IntegrationTest):
+
+  # Ensure Chrome does not use DataSaver in Incognito mode.
+  def testCheckPageWithIncognito(self):
+    with TestDriver() as t:
+      t.AddChromeArg('--enable-spdy-proxy-auth')
+      t.AddChromeArg('--incognito')
+      t.LoadURL('http://check.googlezip.net/test.html')
+      for response in t.GetHTTPResponses():
+        self.assertNotHasChromeProxyViaHeader(response)
+
+if __name__ == '__main__':
+  IntegrationTest.RunAllTests()