DevTools: Make collecting async stacks a bit faster.

This speeds up the example described in the bug from 2.7 sec to 2.0 sec.

BUG=340285
R=yurys@chromium.org, yurys

Review URL: https://codereview.chromium.org/170063004

git-svn-id: svn://svn.chromium.org/blink/trunk@167752 bbb929c8-8fbe-4397-9dbb-9b2b20218538