Remove spurious check for invalid value.

The isCSSPrimitiveValue check is likely covering up parser bugs,
and toCSSPrimitiveValue has the assert to ensure we're not
doing something bad.

R=eae,ksakamoto@chromium.org,timloh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/184103034

git-svn-id: svn://svn.chromium.org/blink/trunk@168577 bbb929c8-8fbe-4397-9dbb-9b2b20218538