Modify the rebaselining logic to handle skipped tests.

One of the first steps when rebaselining results for a test
is to push the existing baseline further down in the tree so that
other ports are not affected when the baseline is updated. However,
if a port further down in the tree skips the test, it doesn't want the
new baseline. Fixing this will keep the Android port from getting a lot
of unwanted baselines while we're bringing back up to date.

R=ojan@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23135008

git-svn-id: svn://svn.chromium.org/blink/trunk@156285 bbb929c8-8fbe-4397-9dbb-9b2b20218538