Revert 168577 "Remove spurious check for invalid value."

> 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

BUG=350509
TBR=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/203583004

git-svn-id: svn://svn.chromium.org/blink/trunk@169456 bbb929c8-8fbe-4397-9dbb-9b2b20218538