Web Animations API: Allow null target in Animation constructor

The IDL spec for Animation declares the Animation target to be nullable.
http://dev.w3.org/fxtf/web-animations/#the-animation-interface
This change makes it possible to specify null for the target parameter
of the Animation constructor.

Review URL: https://codereview.chromium.org/240073004

git-svn-id: svn://svn.chromium.org/blink/trunk@172111 bbb929c8-8fbe-4397-9dbb-9b2b20218538