Revert 162241 "Revert 162178 "Web Animations CSS: Adding ASSERT ..."

Not this one. Relanding.

> 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

TBR=zmo@chromium.org

Review URL: https://codereview.chromium.org/75653003

git-svn-id: svn://svn.chromium.org/blink/trunk@162246 bbb929c8-8fbe-4397-9dbb-9b2b20218538