Make RenderLayerBacking get the timingFunction of the correct animation.

When an element has multiple animations that have different timingFunctions,
the progress of each animation should be calculated using its respective
timingFunction. But at this point, the timingFunction of the first animation is
only used in RenderLayerBacking::startAnimation(), regardless of how many
animations the element has.

This CL makes startAnimation() choose timingFunction by its name.

BUG=288561
TEST=animations/keyframe-multiple-timing-functions-transform.html

Review URL: https://chromiumcodereview.appspot.com/24089002

git-svn-id: svn://svn.chromium.org/blink/trunk@157586 bbb929c8-8fbe-4397-9dbb-9b2b20218538