Stop trying to merge flaky test expectations in update-flaky-tests.

The code in test_expectations.py is really not designed to merge
lines for multiple ports, unfortunately, and we're getting it wrong.

For now, we'll stop trying to be clever and just list one config
per line.

R=eseidel@chromium.org
BUG=393508

Review URL: https://codereview.chromium.org/397843003

git-svn-id: svn://svn.chromium.org/blink/trunk@178227 bbb929c8-8fbe-4397-9dbb-9b2b20218538