DevTools: Fix async stacks for console appended at wrong time.

An async call stack should be appended to a current call stack at the time
when it's collected.

BUG=169721
R=yurys@chromium.org, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/383343003

git-svn-id: svn://svn.chromium.org/blink/trunk@178153 bbb929c8-8fbe-4397-9dbb-9b2b20218538