Don't use nonstandard %% dimension in unit tests

The current parser allows an arbitrary number of %s (e.g. left: 10%%%),
but we shouldn't depend on it in our unit tests.

Review URL: https://codereview.chromium.org/641343004

git-svn-id: svn://svn.chromium.org/blink/trunk@183857 bbb929c8-8fbe-4397-9dbb-9b2b20218538