Looks like it has something to do with background color leaking from a previous test. I tried to to run the test 100 times on the same runner to see whenever the test itself was causing the flakiness and it passed. Unlikely to be related to machine load since it is a http test and runs on a locked shard.
Created attachment 140912 [details]
Actually I could make the test fail by repeatedly running it. That means the flakiness is not caused by other test.
./Tools/Scripts/run-webkit-tests --efl --debug --repeat-each=5000 http/tests/css/shared-stylesheet-mutation.html
Created attachment 140945 [details]
Removing dependency, bot turns red only for failures, not flakes.
Comment on attachment 140945 [details]
Clearing flags on attachment: 140945
Committed r116535: <http://trac.webkit.org/changeset/116535>
All reviewed patches have been landed. Closing bug.