Make sure begin and end events are dispatched for back-to-back intervals

When the end time of the previous interval is the same as the
begin time of the new/current interval, the active state won't 
visibly transition Inactive -> Active (for 'begin' - the other
way around for 'end') if the interval can be resolved up
front (i.e. isn't event-base).

Fix by having SVGSMILElement::checkRestart return a bool indicating
whether an interval ended up being restarted, and then use this bool to
extend the predicates looking for Inactive<->Active transitions.

BUG=337724

Review URL: https://codereview.chromium.org/141013017

git-svn-id: svn://svn.chromium.org/blink/trunk@166625 bbb929c8-8fbe-4397-9dbb-9b2b20218538