Don't write out baselines for tests that are expected to be missing them.

Since adding NeedsRebaseline/NeedsManualRebaseline is standard practice, there will often
be times when tests are committed that are missing baselines.
This allows us to avoid cluttering everyone's checkouts with
untracked baselines for these tests.

Review URL: https://chromiumcodereview.appspot.com/5720827718795264

git-svn-id: svn://svn.chromium.org/blink/trunk@154560 bbb929c8-8fbe-4397-9dbb-9b2b20218538