Web Animations CSS: Avoid starting transitions when interpolation is not defined

r160053[1] did this for interpolation with with AnimatableUnknown. This change
avoids starting transitions for all cases where we don't support smooth
interpolation, eg. between gradients specified for background-image.

This avoids a behavior change where the legacy animations engine would step
such cases at 0. Now, by not starting transitions we ensure that the style
change takes place immediately.

background-image and webkit-mask-image interpolation tests are rebaselined
as we no longer interpolate repeatable lists of images containing gradients.
flex-transitions is updated as we no longer start transitions for flex-grow and
flex-shrink when one of the values is 0 (per spec).

[1] https://src.chromium.org/viewvc/blink?view=rev&revision=160053

BUG=303430

Review URL: https://codereview.chromium.org/115303005

git-svn-id: svn://svn.chromium.org/blink/trunk@163992 bbb929c8-8fbe-4397-9dbb-9b2b20218538