Convert SVGSMILElement::resolveInterval to return an SMILInterval

Rather than using two out-/reference parameters to return the result, use
the return value.
Also convert the 'bool first' argument to the method to an enumeration.

With the above SVGSMILElement::resolve{First,Next}Interval can be
simplified a bit.

Review URL: https://codereview.chromium.org/306293005

git-svn-id: svn://svn.chromium.org/blink/trunk@175321 bbb929c8-8fbe-4397-9dbb-9b2b20218538