Web Animations API: Bindings for TimedItem.specified with readonly attributes

This CL exposes TimedItem.specified (experimental), and getters for each of its attributes.

Follow up patch will add setters.

Follow up patch will refactor support for duration if it is possible to define an attribute with a union type in the current WebIDL implementation.

NOTRY=true
BUG=334927,257235

Review URL: https://codereview.chromium.org/152853003

git-svn-id: svn://svn.chromium.org/blink/trunk@167166 bbb929c8-8fbe-4397-9dbb-9b2b20218538