Revert 162178 "Web Animations CSS: Adding ASSERT for the timing ..."

Suspect this caused fast/frames/seamless/seamless-nested-crash.html to timeout on debug build.

Will reland if my suspicion is incorrect.

> 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

TBR=mithro@mithis.com

Review URL: https://codereview.chromium.org/72043006

git-svn-id: svn://svn.chromium.org/blink/trunk@162241 bbb929c8-8fbe-4397-9dbb-9b2b20218538