DevTools: Stop collecting async stacks while profiling.

BUG=367135
R=pfeldman, yurys, sergeyv

Review URL: https://codereview.chromium.org/257453005

git-svn-id: svn://svn.chromium.org/blink/trunk@172762 bbb929c8-8fbe-4397-9dbb-9b2b20218538