Don't use getPropertyCSSValue in treeoutline.js

As the exposed interface getPropertyCSSValue is going to be removed, we
shouldn't rely on it.

BUG=428595

Review URL: https://codereview.chromium.org/690893002

git-svn-id: svn://svn.chromium.org/blink/trunk@184630 bbb929c8-8fbe-4397-9dbb-9b2b20218538