Web Animations CSS: Adding ASSERT for the timing function in Timing object.

Alternative version would be to populate the timing function in the contructor
with something like this;

BUG=

Review URL: https://codereview.chromium.org/70233007

git-svn-id: svn://svn.chromium.org/blink/trunk@162178 bbb929c8-8fbe-4397-9dbb-9b2b20218538