Web Animations API: Implement setters for properties of TimedItemTiming

NOTRY=true
BUG=334927,257235

Review URL: https://codereview.chromium.org/148293021

git-svn-id: svn://svn.chromium.org/blink/trunk@167234 bbb929c8-8fbe-4397-9dbb-9b2b20218538