Add a script to bisect layout test ordering failures.

Given a list of tests, with the last one failing, cut out half the tests
and rerun until we've hit all the test required to cause the test failure.

It's not clear how much use this script will get yet, and it's pretty small,
so I'm inclined not to spend the time yet to write test for this.

NOTRY=true

Review URL: https://codereview.chromium.org/26868009

git-svn-id: svn://svn.chromium.org/blink/trunk@159962 bbb929c8-8fbe-4397-9dbb-9b2b20218538