Defer first tick to end of timer frame.

Rather than interleave the initialization of transition tweens with their first
tick, defer the first tick to the end of the current timer frame such that any
DOM modifications that happen during the first tick do not trigger a forced
relayout when another tween is initialized (e.g., getComputedStyle).
Shorten.
Ensure that the deferred tick runs this frame.

Previously it was using Date.now(), which might be a millisecond or two past the
now time of the current frame, and thus might not run until the next frame! By
using the current time, we guarantee that it runs at the end of this frame.
Fix transition test.