Simplify CRC2D winding rule handling

Since the bindings already check the enumeration, there's no need to
check that the value passed is valid. Make the parseWinding helper function
just return the translated value instead of a status boolean.

Review URL: https://codereview.chromium.org/226733004

git-svn-id: svn://svn.chromium.org/blink/trunk@171438 bbb929c8-8fbe-4397-9dbb-9b2b20218538