Supply CSS transition easing when creating keyframes for CSS transitions

Linear easing was being supplied instead of the timing function specified by the user.

Example for testing: http://jsbin.com/sihif/1/quiet

Cubic bezier keyframe easings were being calculated without knowledge of the iteration duration. This led to inconsistency in the accuracy being requested.

We now always use the iteration duration when choosing the accuracy for timing function evaluation.

Without this, the transitions layout tests opacity-transform-transitions-inside-iframe.html and
cubic-bezier-overflow-length.html would have needed rebaselining.

BUG=352919

Review URL: https://codereview.chromium.org/196413030

git-svn-id: svn://svn.chromium.org/blink/trunk@169897 bbb929c8-8fbe-4397-9dbb-9b2b20218538