Adds proxy pingback chromedriver integration test (testPingback).

tools/chrome_proxy/webdriver/smoke.py

Index: tools/chrome_proxy/webdriver/smoke.py
diff --git a/tools/chrome_proxy/webdriver/smoke.py b/tools/chrome_proxy/webdriver/smoke.py
index b54dd7fc22424559aff65f2649f7890f10a07180..1b0a8e2ef9d292fc586db5756c7d5db652ffbbd0 100644
--- a/tools/chrome_proxy/webdriver/smoke.py
+++ b/tools/chrome_proxy/webdriver/smoke.py
@@ -31,5 +31,19 @@ class Smoke(IntegrationTest):
       for response in responses:
         self.assertHasChromeProxyViaHeader(response)
 
+  # Ensure pageload metric pingback with DataSaver.
+  def testPingback(self):
+    with TestDriver() as t:
+      t.AddChromeArg('--enable-spdy-proxy-auth')
+      t.AddChromeArg('--enable-data-reduction-proxy-force-pingback')
+      t.LoadURL('http://check.googlezip.net/test.html')
+      t.LoadURL('http://check.googlezip.net/test.html')
+      t.SleepUntilHistogramHasEntry("DataReductionProxy.Pingback.Succeeded")
+      # Verify one pingback attempt that was successful.
+      attempted = t.GetHistogram('DataReductionProxy.Pingback.Attempted')
+      self.assertEqual(1, attempted['count'])
+      succeeded = t.GetHistogram('DataReductionProxy.Pingback.Succeeded')
+      self.assertEqual(1, succeeded['count'])
+
 if __name__ == '__main__':
   IntegrationTest.RunAllTests()