Make Element::ensureActiveAnimations() return a reference

Make Element::ensureActiveAnimations() return a reference instead of a pointer
as it can never return null. This makes the calling code look safer.

R=adamk

Review URL: https://codereview.chromium.org/182293002

git-svn-id: svn://svn.chromium.org/blink/trunk@167973 bbb929c8-8fbe-4397-9dbb-9b2b20218538