[css3-transitions] Switch TransitionEvent to use a constructor rather than an init* method

Currently, the IDL for TransitionEvent uses the init*() method pattern
for manually creating events.  This pattern has been generally
repudiated in favor of simple constructors, so we should switch
TransitionEvent over to that.

Interestingly, AnimationEvent has already been changed to use a
constructor, so it looks like this would really just be a consistency
sync-up.

Even more interestingly, it looks like WebKit has already switched its
code to use a constructor, as evidenced by this recent test check-in:
<http://trac.webkit.org/browser/trunk/LayoutTests/fast/events/constructors/transition-event-constructor.html?rev=140448>

So, someone mind doing a quick edit of the spec's IDL to fix this issue?

~TJ