Adding expectation for flaky performance test

We've seen this test pass intermittently on Linux and now MacOS so
I am adding an expectation for it until the test owner can investigate
and fix.

TBR=npm@chromium.org

Bug: 896765
Change-Id: Icc45010eb16f298a5587404b9530853fca55349a
Reviewed-on: https://chromium-review.googlesource.com/c/1289961Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600974}

third_party/WebKit/LayoutTests/TestExpectations

@@ -5333,3 +5333,6 @@ crbug.com/891155 [ Linux ] fast/events/middleClickAutoscroll-drag.html [ Failure
 
 # ecosystem-infra sheriff 2018-10-16
 crbug.com/895777 external/wpt/infrastructure/assumptions/blank.html [ Failure Crash ]
+
+#Sheriff 2018-10-18
+crbug.com/896765 [ Linux Mac ] http/tests/performance-timing/element-timing/observe-child-element.html [ Failure Pass ]
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