Dont try to schedule animation when the frame has been detached.

No new tests. Depends on a race which is difficult to test. Will monitor canary.

TBR=steveblock
BUG=259447
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/18683005

git-svn-id: svn://svn.chromium.org/blink/trunk@154147 bbb929c8-8fbe-4397-9dbb-9b2b20218538