Make some property parsing helpers return CSSValues instead of adding directly

Currently many of the helper functions in CSSPropertyParser will parse
a value and directly add it to the list of parsed properties. It is
somewhat nicer if these instead return a CSSValue, so these helper
functions won't need to know about whether the rule is marked as
important.

BUG=404023

Review URL: https://codereview.chromium.org/499173002

git-svn-id: svn://svn.chromium.org/blink/trunk@180889 bbb929c8-8fbe-4397-9dbb-9b2b20218538