Avoid exposing disabled CSS properties in will-change and transition

Both transition and will-change should produce string values for unknown
properties, but were producing identifiers. transition requires follow on
work as it only currently considers valid property names.

NOTRY=true

Review URL: https://codereview.chromium.org/216003002

git-svn-id: svn://svn.chromium.org/blink/trunk@170286 bbb929c8-8fbe-4397-9dbb-9b2b20218538