DevTools: [Styles] Make sure autocompleted property value gets into the stylesheet

We've had a race between the delayed autocompletion and live stylesheet updates,
which used the non-completed property value in certain cases, and it never
made its way into the stylesheet.

R=aandrey, eustas, vsevik
BUG=326393

Review URL: https://codereview.chromium.org/118473004

git-svn-id: svn://svn.chromium.org/blink/trunk@164313 bbb929c8-8fbe-4397-9dbb-9b2b20218538