Ensure CSSParserValue::fValue is valid when parsing numbers

Calls to validUnit(v, FNumber) were incorrectly reading v->fValue
when a calc() was present. This change ensures number calcs get
resolved in validUnit() and stored in v->fValue as a CSS_NUMBER.
This change allows CSS property parsing to continue to validate
the input number even if it is a calc expression.

BUG=395972

Review URL: https://codereview.chromium.org/412133002

git-svn-id: svn://svn.chromium.org/blink/trunk@179293 bbb929c8-8fbe-4397-9dbb-9b2b20218538