Protect readystatechange event dispatch on XMLHttpRequest.

Protect readystatechange event dispatch on XMLHttpRequest.

XMLHttpRequestProgressEventThrottle::dispatchReadyStateChangeEvent may dispatch
multiple events: a deferred progress event and the given ready state change.
Multiple dispatching may lead to a use-after-free bug.

This CL adds a protection for the event target. In addition to that,
XMLHttpRequestProgressEventThrottle stops dispatching the latter event when
dispatching the former event changes the readyState.

BUG=505362
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197983

[yhirano, 2015-06-29 10:33:46]

yhirano@chromium.org changed reviewers:
+ sigbjornf@opera.com

[sof, 2015-06-29 11:00:27]

lgtm

https://codereview.chromium.org/1217803002/diff/1/Source/core/xmlhttprequest/...
File Source/core/xmlhttprequest/XMLHttpRequestProgressEventThrottle.cpp (right):

https://codereview.chromium.org/1217803002/diff/1/Source/core/xmlhttprequest/...
Source/core/xmlhttprequest/XMLHttpRequestProgressEventThrottle.cpp:112: // We
don't dispatch the event when an event handler associated with
This repeats in prose what the check above does; it would be more interesting to
state why.

[yhirano, 2015-06-29 11:43:47]

https://codereview.chromium.org/1217803002/diff/1/Source/core/xmlhttprequest/...
File Source/core/xmlhttprequest/XMLHttpRequestProgressEventThrottle.cpp (right):

https://codereview.chromium.org/1217803002/diff/1/Source/core/xmlhttprequest/...
Source/core/xmlhttprequest/XMLHttpRequestProgressEventThrottle.cpp:112: // We
don't dispatch the event when an event handler associated with
On 2015/06/29 11:00:27, sof wrote:
> This repeats in prose what the check above does; it would be more interesting
to
> state why.

Done.

[yhirano, 2015-06-29 11:43:55]

The CQ bit was checked by yhirano@chromium.org

[yhirano, 2015-06-29 11:43:55]

The patchset sent to the CQ was uploaded after l-g-t-m from sigbjornf@opera.com
Link to the patchset: https://codereview.chromium.org/1217803002/#ps20001
(title: " ")

[commit-bot: I haz the power, 2015-06-29 11:44:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1217803002/20001

[commit-bot: I haz the power, 2015-06-29 12:38:54]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197983