Mark http/tests/cache/zero-length-xhr.html test flaky

Bug: 1066716
Change-Id: I7dfe445604ad6e8556e7f9de462ceed7f0305fd1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2142883Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758154}

third_party/blink/web_tests/TestExpectations

@@ -6661,6 +6661,9 @@ crbug.com/1066732 fast/events/platform-wheelevent-paging-y-in-scrolling-div.html
 # These tests will pass once the --enable-features=TrustTokens flag is on by default.
 crbug.com/1061765 external/wpt/trust-tokens/end-to-end/* [ Skip ]
 
+# Test flakes
+crbug.com/1066716 http/tests/cache/zero-length-xhr.html [ Pass Timeout ]
+
 # Sheriff 2020-04-04
 crbug.com/1067533 [ Mac10.10 ] external/wpt/shape-detection/idlharness.https.any.sharedworker.html [ Pass Failure ]