Mark a layout test virtual/threaded/fast/compositorworker/visual-update.html as flaky

Bug: 727991
Change-Id: Ife0e27562727cf08ff40615baf3f024ed621534d
Reviewed-on: https://chromium-review.googlesource.com/518745Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Takeshi Yoshino <tyoshino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#475836}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3036,3 +3036,6 @@ crbug.com/727257 [ Mac10.11 ] svg/animations/animate-local-url.html [ Pass Failu
 # Sheriff failures 2017-05-30
 # Failing on try (with patch) and Linux FYI too frequently.
 crbug.com/727505 [ Linux ] virtual/threaded/fast/compositorworker/basic-plumbing-main-to-worker.html [ Pass Crash ]
+
+# Sheriff failures 2017-05-31
+crbug.com/727991 [ Linux Mac ] virtual/threaded/fast/compositorworker/visual-update.html [ Pass Failure ]