Update testLitePageBTFWithoutFallback to use "alt6" experiment

The test testLitePageBTFWithoutFallback is failing due passing in the "alt1" experiment which is the basic team food experiment and will return a stale preview.

The test is expecting a lite page, the experiment needs to be changed to alt6 which will force lite page.

Bug: 822863
Change-Id: I657ab3d08c6d40c224ca190434eb37bdf1a102d0
Reviewed-on: https://chromium-review.googlesource.com/967147Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Richard Bustamante <bustamante@google.com>
Cr-Commit-Position: refs/heads/master@{#543805}

tools/chrome_proxy/webdriver/lite_page.py

@@ -209,8 +209,8 @@ class LitePage(IntegrationTest):
       test_driver.AddChromeArg('--force-effective-connection-type=2G')
 
       # Need to force lite page so target page doesn't fallback to Lo-Fi
-      # Set exp=alt1 to force Lite-page response.
-      test_driver.AddChromeArg('--data-reduction-proxy-experiment=alt1')
+      # Set exp=alt6 to force Lite-page response.
+      test_driver.AddChromeArg('--data-reduction-proxy-experiment=alt6')
 
       # This page is long and has many media resources.
       test_driver.LoadURL('http://check.googlezip.net/metrics/index.html')