TimingFunction test helper.

Makes it easier to use TimingFunction's in your tests as they can now be pretty
printed. (They already had an operator==.)

BUG=

Review URL: https://codereview.chromium.org/39223002

git-svn-id: svn://svn.chromium.org/blink/trunk@161306 bbb929c8-8fbe-4397-9dbb-9b2b20218538