Add some ASSERTS to TimingFunction::evaluate()

The current animations implementation should never use time fractions outside
[0, 1]. The Web Animations model will allow time fractions outside this range,
but the behavior in this case is not yet specified or implemented.

This change adds asserts to make sure this invariant is mainted as we add
support for chained timing functions.

BUG=240653
R=dstockwell

Review URL: https://chromiumcodereview.appspot.com/23621024

git-svn-id: svn://svn.chromium.org/blink/trunk@157576 bbb929c8-8fbe-4397-9dbb-9b2b20218538