Rename CSSPrimitiveValue::UnitTypes to CSSPrimitiveValue::UnitType

Rename CSSPrimitiveValue::UnitTypes to CSSPrimitiveValue::UnitType.
Remove unnecessary uses of unsigned short that represent a
CSSPrimitiveValue::UnitType, in particular the return value of
CSSPrimitiveValue::primitiveType().

Review URL: https://codereview.chromium.org/298823003

git-svn-id: svn://svn.chromium.org/blink/trunk@174674 bbb929c8-8fbe-4397-9dbb-9b2b20218538