style: Implement the smarter interporation for transform.

Corresponding to this spec change;
w3c/csswg-drafts@3281266

The expected value in test_transitions_per_property.html can be calculated;

  'start' + ('end' - 'start') * 0.25

Bug: 1464647
Reviewed-by: birtles, emilio
MozReview-Commit-ID: NI9gOUuPnG

Hiroyuki Ikezoe authored and emilio committed Jun 2, 2018

378fcc2