Convert SVGSMILElement::m_interval{Begin,End} into a SMILInterval

This is a mostly mechanical change that makes use of the SMILInterval
type added in https://codereview.chromium.org/298043005.

Review URL: https://codereview.chromium.org/312503003

git-svn-id: svn://svn.chromium.org/blink/trunk@175285 bbb929c8-8fbe-4397-9dbb-9b2b20218538