Have rebaseline-expectations consider one expectations file at a time.

This simplifies the semantics of the Rebaseline keyword in a future world
in which we merge different expectations files instead of having one
overwrite the other.

Review URL: https://chromiumcodereview.appspot.com/20213003

git-svn-id: svn://svn.chromium.org/blink/trunk@154969 bbb929c8-8fbe-4397-9dbb-9b2b20218538