DevTools: Revert moving async stack traces behind an experiment because it is done in incomplete way and is hard to merge.

R=pfeldman@chromium.org
TBR=pfeldman

Review URL: https://codereview.chromium.org/159753004

git-svn-id: svn://svn.chromium.org/blink/trunk@166932 bbb929c8-8fbe-4397-9dbb-9b2b20218538