Refactor AsyncCallStackTracker to hide private implementation inner class.

R=yurys, haraken, keishi, oilpan-reviews

Review URL: https://codereview.chromium.org/707273003

git-svn-id: svn://svn.chromium.org/blink/trunk@185141 bbb929c8-8fbe-4397-9dbb-9b2b20218538