The CrExtensionsItemListTest.NoItems and CrExtensionsItemListTest.Filtering test cases of browser_tests are flaky on Mac10.9 Tests (dbg)

It appears it started with this build
https://build.chromium.org/p/chromium.mac/builders/Mac10.9%20Tests%20%28dbg%29/builds/45659
but I cannot find the culprit. It also has passed since, but most of the time there's a failure on one of the tests. 

Example failure log:
11291:771:1002/220611.686671:INFO:CONSOLE(1220)] "Running TestCase CrExtensionsItemListTest.Filtering", source: test_api.js (1220)
[11291:771:1002/220612.443400:ERROR:CONSOLE(55)] "Uncaught TypeError: Cannot read property 'observe' of null", source: chrome://resources/cr_elements/cr_container_shadow_behavior.js (55)
[11291:771:1002/220612.458069:ERROR:web_ui_browser_test.cc(467)] CONDITION FAILURE: encountered javascript console error(s):
[11291:771:1002/220612.458114:ERROR:web_ui_browser_test.cc(469)] JS ERROR: '[11291:771:1002/220612.443400:ERROR:CONSOLE(55)] "Uncaught TypeError: Cannot read property 'observe' of null", source: chrome://resources/cr_elements/cr_container_shadow_behavior.js (55)
'
[11291:771:1002/220612.458137:ERROR:web_ui_browser_test.cc(471)] JS call assumed failed, because JS console error(s) found.
gen/chrome/test/data/webui/extensions/cr_extensions_browsertest-gen.cc:638: Failure
Value of: RunJavascriptTestF( true, "CrExtensionsItemListTest", "Filtering")
  Actual: false
Expected: true