Mark linebreak_simple_a.html as flaky on cpu reftests.

It has failed on three unrelated PRs:

https://s3.amazonaws.com/archive.travis-ci.org/jobs/31574326/log.txt
https://s3.amazonaws.com/archive.travis-ci.org/jobs/31581185/log.txt
https://s3.amazonaws.com/archive.travis-ci.org/jobs/31592052/log.txt