DevTools: Support XHR async call stacks in the debugger.

With this we will collect XHR async call stacks at the point of
calling XHR.send() rather than at XHR.addEventListener() or at
assigning XHR.onreadystatechange.

For XMLHttpRequestUpload event target callbacks show async call stacks
of the owner XHR object.

Drive-by: Do strict counting of the nested willHandleEvent callbacks.

BUG=272416
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/118293002

git-svn-id: svn://svn.chromium.org/blink/trunk@164291 bbb929c8-8fbe-4397-9dbb-9b2b20218538