Reset animation player map on disconnecting Inspector frontend.

Clear out the node ID to AnimationPlayer map on disconnect to avoid
references to elements (and their documents) from lingering longer than
necessary.

R=haraken,aandrey
BUG=428685

Review URL: https://codereview.chromium.org/690603003

git-svn-id: svn://svn.chromium.org/blink/trunk@184743 bbb929c8-8fbe-4397-9dbb-9b2b20218538