Allocate a singleton LinearTimingFunction

LinearTimingFunction has no state. We can allocate a single instance instead of creating a new instance each time linear timing is requested for an animation.

We already have singletons for ease, ease-in, ease-out, ease-in-out.

BUG=350998

Review URL: https://codereview.chromium.org/189543016

git-svn-id: svn://svn.chromium.org/blink/trunk@168979 bbb929c8-8fbe-4397-9dbb-9b2b20218538