Have Frame::animation() return a reference

Have Frame::animation() return a reference instead of a pointer as it
can never return null. This makes the calling code look safer and gets
rid a several unnecessary null-checks.

BUG=

Review URL: https://codereview.chromium.org/33483002

git-svn-id: svn://svn.chromium.org/blink/trunk@160166 bbb929c8-8fbe-4397-9dbb-9b2b20218538