Clear current target in TimelineModelImpl only when recording has stopped

reset can be called when clearing records in Timeline that is being recorded. After such action recording continues and current Target doesn't change.

BUG=397216
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/417123002

git-svn-id: svn://svn.chromium.org/blink/trunk@178907 bbb929c8-8fbe-4397-9dbb-9b2b20218538