Mark heap-snapshot-with-detached-dom-tree.js as flaky on Mac

It has been timing out flakily and causing some CQ false rejections.

Bug: 1027436
Change-Id: I093ef6e1aeed38d36488825dbea1181e9afafcae
No-Try: True
TBR: petermarshall@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1944353Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720214}

third_party/blink/web_tests/TestExpectations

@@ -5851,6 +5851,9 @@ crbug.com/1028684 inspector-protocol/animation/animation-release.js [ Pass Failu
 crbug.com/1015187 virtual/cross-origin-isolation/external/wpt/html/cross-origin-embedder-policy/none.https.html [ Pass Failure ]
 crbug.com/1015187 [ Linux Mac ] external/wpt/html/cross-origin-embedder-policy/none.https.html [ Pass Failure Timeout ]
 
+# Sheriff 2019-11-29
+crbug.com/1027436 [ Mac ] inspector-protocol/heap-profiler/heap-snapshot-with-detached-dom-tree.js [ Pass Timeout ]
+
 # Temporary suppression to allow Skia change to opaqueness tracking
 crbug.com/1028116 virtual/gpu-rasterization-disable-yuv/images/yuv-decode-eligible/color-profile-border-radius.html [ Pass Failure ]
 crbug.com/1028116 virtual/gpu-rasterization-disable-yuv/images/yuv-decode-eligible/color-profile-filter.html [ Pass Failure ]