Use devtools to set user agent in chromedriver

chrome/test/chromedriver/test/run_py_tests.py

Index: chrome/test/chromedriver/test/run_py_tests.py
diff --git a/chrome/test/chromedriver/test/run_py_tests.py b/chrome/test/chromedriver/test/run_py_tests.py
index 2c5e728952ef44f8938b54e24d74f51f13a94c72..c64d862debed30e4ef5c4a167a2191c15796cfab 100755
--- a/chrome/test/chromedriver/test/run_py_tests.py
+++ b/chrome/test/chromedriver/test/run_py_tests.py
@@ -1836,7 +1836,7 @@ class ChromeLogPathCapabilityTest(ChromeDriverBaseTest):
     self.assertTrue(self.LOG_MESSAGE in open(tmp_log_path.name).read())
 
 
-class MobileEmulationCapabilityTest(ChromeDriverBaseTest):
+class MobileEmulationCapabilityTest(ChromeDriverBaseTestWithWebServer):
   """Tests that ChromeDriver processes chromeOptions.mobileEmulation.
 
   Makes sure the device metrics are overridden in DevTools and user agent is
@@ -1905,6 +1905,20 @@ class MobileEmulationCapabilityTest(ChromeDriverBaseTest):
     body_tag = driver.FindElement('tag name', 'body')
     self.assertEqual("Agent Smith", body_tag.GetText())
 
+  def testUserAgentNewTab(self):
+    driver = self.CreateDriver(
+        mobile_emulation = {'userAgent': 'Agent Smith'})
+    driver.Load(self.GetHttpUrlForFile('/chromedriver/newtab.html'))
+    old_windows = driver.GetWindowHandles()
+    link = driver.FindElements('tag name', 'a')[0]
+    driver.MouseMoveTo(link)
+    driver.MouseClick()

[stgao, 2017/04/05 04:17:08]

What's the user-agent if the new tab itself directly loads the "/userAgent"
below before Devtools connects to the new tab and instructs it to simulate the
user-agent?

[irisu, 2017/04/05 06:44:33]

Sorry for that misunderstanding. Modified the test and you are correct - this
test now fails.

I will try looking at the docs for a fix, otherwise it may be best to add the
user-agent flag to headless after all to avoid this.

[Sami, 2017/04/05 13:52:04]

Right, looks like we need to support a default user agent after all for this
reason.

+    new_window = self.WaitForNewWindow(driver, old_windows)
+    driver.SwitchToWindow(new_window)
+    driver.Load(self._http_server.GetUrl() + '/userAgent')

[stgao, 2017/04/05 04:17:08]

Before this load, we already instruct the new tab to simulate the user-agent
above, so this load will return the simulated one for sure.

I'm curious about the user-agent of the page that the new tab loads by itself
when opened.

+    body_tag = driver.FindElement('tag name', 'body')
+    self.assertEqual("Agent Smith", body_tag.GetText())
+
   def testDeviceName(self):
     driver = self.CreateDriver(
         mobile_emulation = {'deviceName': 'Google Nexus 5'})