Remove TimelineRecordImple.embedderCallbackName field

Use record data to retrieve its value when needed.

BUG=None
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/294033004

git-svn-id: svn://svn.chromium.org/blink/trunk@174450 bbb929c8-8fbe-4397-9dbb-9b2b20218538