Unflake testTCPReset

The WarmupURLFetcherCallback histogram could get recorded more number of
times depending on retries.

Bug: 969795
Change-Id: I13b65dfe4a0fd7a70047f912ff960c201c2eb719
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1641811
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Commit-Queue: rajendrant <rajendrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665816}

tools/chrome_proxy/webdriver/proxy_connection.py

@@ -23,7 +23,7 @@ def VerifyWarmupHistogram(self, test_driver, is_secure_proxy):
         histogram_part)
       histogram = test_driver.GetBrowserHistogram(histogram_name)
       if histogram:
-        self.assertEqual(1, histogram['count'])
+        self.assertLessEqual(1, histogram['count'])
         is_histogram_found = True
     self.assertTrue(is_histogram_found)