Remove _callSiteStackTrace field from TimelineRecordImpl

callSiteStackTrace should always return async initiator's stack trace.

BUG=None

Review URL: https://codereview.chromium.org/299683003

git-svn-id: svn://svn.chromium.org/blink/trunk@174386 bbb929c8-8fbe-4397-9dbb-9b2b20218538