Fix for Assert and more with bad CSS input.

The CSS input had a strange belief that users are nice and would not
feed it strange strings. This caused asserts and bug 360403.

BUG=360403

Review URL: https://codereview.chromium.org/230173005

git-svn-id: svn://svn.chromium.org/blink/trunk@171169 bbb929c8-8fbe-4397-9dbb-9b2b20218538