Web Animations: Fix potential crash if a Player outlives its Timeline

This patch fixes a potential crash if a Player outlives the Timeline
it is attached to. Probably the correct behaviour here is to keep the
Timeline alive, but this isn't particularly easy due to the reference
cycle. Hopefully we can fix this properly once Oilpan comes around.

NOTRY=true
BUG=334937

Review URL: https://codereview.chromium.org/169163003

git-svn-id: svn://svn.chromium.org/blink/trunk@167265 bbb929c8-8fbe-4397-9dbb-9b2b20218538