Ensure litepages probe completes before running ChromeDriver test

Bug: 1049675
Change-Id: Ide9d8a6fccbb53573d57c3232de2f7d5d3986647
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2065349
Auto-Submit: Robert Ogden <robertogden@chromium.org>
Commit-Queue: rajendrant <rajendrant@chromium.org>
Reviewed-by: rajendrant <rajendrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742877}

tools/chrome_proxy/webdriver/https_previews.py

@@ -36,6 +36,10 @@ class HttpsPreviews(IntegrationTest):
     t.AddChromeArg('--ignore-litepage-redirect-optimization-blacklist')
     t.SetExperiment('external_chrome_integration_test')
 
+    # Wait for the server probe to complete before starting the test, otherwise
+    # it will flake.
+    t.SleepUntilHistogramHasEntry('Availability.Prober.FinalState.Litepages')
+
   def _AssertShowingLitePage(self, t, expectedText, expectedImages):
     """Asserts that Chrome has loaded a Lite Page from the litepages server.