Use NaN/Inf to represent unresolved/indefinite SMILTimes.

R=fs@opera.com
BUG=404601

Review URL: https://codereview.chromium.org/478273002

git-svn-id: svn://svn.chromium.org/blink/trunk@180823 bbb929c8-8fbe-4397-9dbb-9b2b20218538