Do not presume that |yValue| is primitive if |value| is implicit in StylePropertySerializer

An implicit y-value can become explicit if specified as a separate longhand.
At the same time, its new value can be non-primitive.

R=alexis.menard@intel.com
BUG=257347

Review URL: https://chromiumcodereview.appspot.com/18710002

git-svn-id: svn://svn.chromium.org/blink/trunk@153678 bbb929c8-8fbe-4397-9dbb-9b2b20218538