FYI mash_ash_unittests == ash_unittest --enable-features=Mash

This sounds like  issue 794703 , another flaky highlighter test that is disabled in the filter file:
https://cs.chromium.org/chromium/src/testing/buildbot/filters/ash_unittests_mash.filter?type=cs&q=highlightercontrollertest&sq=package:chromium&l=88

Obsolete. There's nothing to do here. We've reduced the amount of stuff in ash that changes behavior under --enable-features=Mash, so we eliminated the mash_ash_unittests test suite.