Web Animations: Remove activeDuration

This was removed from the spec [1] in favor of endDelay.

[1] https://github.com/web-animations/web-animations-spec/commit/d00254dd8a79ae5f06b3db144f37302bda4f69ce
NOTRY=true
BUG=335350

Review URL: https://codereview.chromium.org/180163003

git-svn-id: svn://svn.chromium.org/blink/trunk@168263 bbb929c8-8fbe-4397-9dbb-9b2b20218538