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
Merge pull request #2997 from Ms2ger/flaky-reftest

Mark linebreak_simple_a.html ass flaky on cpu reftests; r=SimonSapin