There's no need to have a virtual method here when we can just store the
owning element pointer on the Animation instead.
(cherry picked from commit a0b96280e498d53576ec855d0deea56f508c9e7c)
Now that EasingStyleValue is a lot nicer to use, there isn't much reason
to keep TimingFunction around.
(cherry picked from commit 7950992fc21e2428a7f32954bbe893a2b2d58cf7,
manually amended with the output of `git clang-format master`)