Issue 896766  has been merged into this issue.

First failures seen on this build:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Linux%20Tests%20%28dbg%29%281%29/75100

The failure is intermittent and looks like it was introduced here:
https://chromium-review.googlesource.com/c/chromium/src/+/1277675

Finally found the error log for the test:

https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8932487525162236112/+/steps/webkit_layout_tests/0/logs/json.output/0:

            "observe-child-element.html": {
              "actual": "TEXT TEXT TEXT TEXT",
              "expected": "PASS",
              "has_stderr": true,
              "is_regression": true,
              "is_testharness_test": true,
              "is_unexpected": true,
              "time": 0.6
            },

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ce5c2317dab655c8913ff31cbee0b438138f1b7f

commit ce5c2317dab655c8913ff31cbee0b438138f1b7f
Author: Joe Downing <joedow@chromium.org>
Date: Fri Oct 19 00:01:03 2018

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/1289961
Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600974}
[modify] https://crrev.com/ce5c2317dab655c8913ff31cbee0b438138f1b7f/third_party/WebKit/LayoutTests/TestExpectations

Removing sheriff label since the intermittent failures should be ignored now.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3d98ac011e7dca78af93f020fc8ff2cf997b4da5

commit 3d98ac011e7dca78af93f020fc8ff2cf997b4da5
Author: Joe Downing <joedow@chromium.org>
Date: Fri Oct 19 17:33:05 2018

Adding windows to TestExpections for observe-child-element.html

TBR=npm@chromium.org

Bug:  896765 
Change-Id: I39b4ece08f31a24ae8536f3c5e4e8fe380073f7b
Reviewed-on: https://chromium-review.googlesource.com/c/1291712
Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601211}
[modify] https://crrev.com/3d98ac011e7dca78af93f020fc8ff2cf997b4da5/third_party/WebKit/LayoutTests/TestExpectations

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/045dade49416490cc8910a3a08dc7aa70340e0fb

commit 045dade49416490cc8910a3a08dc7aa70340e0fb
Author: Nicolas Pena <npm@chromium.org>
Date: Fri Dec 07 18:29:14 2018

[ElementTiming] Fix flaky observe-child-element.html

The timeout used to bail out early is causing flakiness, so this change
removes that timeout since it is not needed for the test. It also
increases the timeout used in cross-origin-iframe-element.html to give
chance for the entry to be dispatched. Since this test passes when such
entry is NOT dispatched, that test is not currently flaky.

Bug: 879270,  896765 
Change-Id: Ie02a9787d721e6c0d1f347b0d08dd9dbd985ca79
Reviewed-on: https://chromium-review.googlesource.com/c/1363960
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614753}
[modify] https://crrev.com/045dade49416490cc8910a3a08dc7aa70340e0fb/third_party/blink/web_tests/TestExpectations
[modify] https://crrev.com/045dade49416490cc8910a3a08dc7aa70340e0fb/third_party/blink/web_tests/http/tests/performance-timing/element-timing/cross-origin-iframe-element.html
[modify] https://crrev.com/045dade49416490cc8910a3a08dc7aa70340e0fb/third_party/blink/web_tests/http/tests/performance-timing/element-timing/observe-child-element.html