DevTools: Fix async stacks instrumentation for XHRs.

This fixes 2 bugs:
- XHR async call stacks could be deleted too early having the second onloadend
  callback w/o async stacks.
- XHR async call stacks were not deleted at all if there was no onloadend event
  listener attached (aka memory leak).

BUG=272416
R=yurys, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/383363002

git-svn-id: svn://svn.chromium.org/blink/trunk@178154 bbb929c8-8fbe-4397-9dbb-9b2b20218538