Improve isolation / mix-blend-mode CSS parsing

The current code was correct but confusing. That's because it
wrongly gated isKeywordPropertyID on the runtime flag.

The right way is to gate the _parsing_ of the properties based
on the runtime flag. Fixed this before someone wrongly copy
this code for a new property.

Review URL: https://codereview.chromium.org/55503003

git-svn-id: svn://svn.chromium.org/blink/trunk@161182 bbb929c8-8fbe-4397-9dbb-9b2b20218538