Updated Client LoFi test to use a https:// test page to reduce flakiness

This should help prevent flakes caused by the server choosing to use
a different type of server-provided preview (e.g. LitePages).

Bug: 738603
Change-Id: I20990a03c167376f319fb521f36341b38ced960d
Reviewed-on: https://chromium-review.googlesource.com/882458Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Commit-Queue: Scott Little <sclittle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531773}

tools/chrome_proxy/webdriver/lofi.py

@@ -442,8 +442,7 @@ class LoFi(IntegrationTest):
       # Verify that Lo-Fi responses were seen.
       self.assertNotEqual(0, lofi_responses)
 
-  # Checks that Client LoFi resource requests have the Intervention header
-  # (in case page has https images that may not be fully loaded).
+  # Checks that Client LoFi resource requests have the Intervention header.
   @ChromeVersionEqualOrAfterM(61)
   def testClientLoFiInterventionHeader(self):
     with TestDriver() as test_driver:
@@ -459,7 +458,7 @@ class LoFi(IntegrationTest):
           '--force-fieldtrials=NetworkQualityEstimator/Enabled/'
           'PreviewsClientLoFi/Enabled')
 
-      test_driver.LoadURL('http://check.googlezip.net/static/index.html')
+      test_driver.LoadURL('https://check.googlezip.net/static/index.html')
 
       intervention_headers = 0
       for response in test_driver.GetHTTPResponses():