DevTools: Fix async stacks instrumentation for XHRs.

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=178154

[aandrey, 2014-07-12 16:21:29]

https://codereview.chromium.org/383363002/diff/20001/LayoutTests/inspector/so...
File LayoutTests/inspector/sources/debugger/async-callstack-events.html (right):

https://codereview.chromium.org/383363002/diff/20001/LayoutTests/inspector/so...
LayoutTests/inspector/sources/debugger/async-callstack-events.html:45:
window.addEventListener("hashchange", onHashChange2, true);
Note: this test would also pass before, just adding this case.

https://codereview.chromium.org/383363002/diff/20001/LayoutTests/inspector/so...
File LayoutTests/inspector/sources/debugger/async-callstack-xhrs-expected.txt
(right):

https://codereview.chromium.org/383363002/diff/20001/LayoutTests/inspector/so...
LayoutTests/inspector/sources/debugger/async-callstack-xhrs-expected.txt:32:
[XMLHttpRequest.send]
Note: here we would have no async stack w/o this patch.

[yurys, 2014-07-14 16:15:41]

lgtm

[aandrey, 2014-07-15 07:47:12]

The CQ bit was checked by aandrey@chromium.org

[commit-bot: I haz the power, 2014-07-15 07:47:26]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/aandrey@chromium.org/383363002/20001

[commit-bot: I haz the power, 2014-07-15 08:49:51]

Change committed as 178154