Compute the keyTimes index correctly for discrete (values) animations

The last value is allowed to be non-one for discrete mode animations (but
not for linear and spline mode animations).

TEST=svg/animations/animate-elem-{14,18}-t-drt.html
BUG=350318

Review URL: https://codereview.chromium.org/197283002

git-svn-id: svn://svn.chromium.org/blink/trunk@169211 bbb929c8-8fbe-4397-9dbb-9b2b20218538