Remove the map in StylePropertyShorthand.

We can just use a simple switch case and create the
Vector<StylePropertyShorthand> on demand (note that the vector
will still be copied to the caller but that was the case before).
This commit will just avoid to have the map always in memory.

Review URL: https://chromiumcodereview.appspot.com/24350013

git-svn-id: svn://svn.chromium.org/blink/trunk@158230 bbb929c8-8fbe-4397-9dbb-9b2b20218538