Parse "-webkit-columns: auto <length>" properly.

The two longhands for the 'columns' property ('column-count' and
'column-width') may both take 'auto' as a value. When we encounter
'auto' during parsing the value list of a declaration, we cannot just
make a guess at which property/properties that's meant for. Instead,
don't assign anything to 'auto' right away, but wait until all values
have been processed and at that point set the still unassigned
properties to 'auto'. If 'auto' isn't in the value list at all, set
unassigned properties to 'initial' for the 'columns' property, just
like we do for any other property.

R=
BUG=235415

Review URL: https://chromiumcodereview.appspot.com/14334014

git-svn-id: svn://svn.chromium.org/blink/trunk@155047 bbb929c8-8fbe-4397-9dbb-9b2b20218538