Oilpan: Reduce RefPtr to WorkerGlobalScope.

Oilpan: Reduce RefPtr to WorkerGlobalScope.

This CL reduces the number of required Heap::collectGarbage calls in
Heap::collectAllGarbage on Worker termination.

 - Make Event::m_target Member<>.
 - Make MessageEvent::m_source Member<>.
  MessageEvent is used to communicate between the main thread and workers.
  For consistency, this CL adjusts MessagePort argument/return types.

They reduce RefPtr to WorkerGlobalScope, which implements EventTarget.

BUG=375671
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175201

[tkent, 2014-05-30 08:52:17]

Please review this.

[haraken, 2014-05-30 12:30:46]

> * Introduce ExecutionContext::deleteLifecycleNotifier(), and 
WorkerGlobalScope::dispose() calls it. 
> Before this change, LifecycleObserver::contextDestroyed was called when a 
> WorkerGlobalScope is destructed. After this change, it is called when a 
> WorkerGlobalScope::dispose is called.

I'd like to understand why it's safe to make this change.

- According to the comment in WorkerGlobalScope::dispose(), at the point where
WorkerGlobalScope::dispose() is called, several objects are still keeping the
WorkerGlobalScope alive to access WorkerGlobalScope::m_thread. This worries me
if we can call contextDestroyed() in WorkerGlobalScope::dispose().

- Given that the main thread calls contextDestroyed() when a Document is
destructed, it seems more consistent if worker threads call contextDestroyed()
when a WorkerGlobalScope is destructed.

https://codereview.chromium.org/304353004/diff/1/Source/core/events/MessageEv...
File Source/core/events/MessageEvent.h (right):

https://codereview.chromium.org/304353004/diff/1/Source/core/events/MessageEv...
Source/core/events/MessageEvent.h:127: explicit
MessageEvent(PassRefPtr<ArrayBuffer> data, const String& origin);

Drop explicits from these constructors.

[tkent, 2014-05-30 14:11:53]

On 2014/05/30 12:30:46, haraken wrote:
> > * Introduce ExecutionContext::deleteLifecycleNotifier(), and 
> WorkerGlobalScope::dispose() calls it. 
> > Before this change, LifecycleObserver::contextDestroyed was called when a 
> > WorkerGlobalScope is destructed. After this change, it is called when a 
> > WorkerGlobalScope::dispose is called.
> 
> I'd like to understand why it's safe to make this change.

According to try bot result, the change was not safe.  I have removed it from
Patch Set 2.

[haraken, 2014-05-30 15:36:42]

On 2014/05/30 14:11:53, tkent wrote:
> On 2014/05/30 12:30:46, haraken wrote:
> > > * Introduce ExecutionContext::deleteLifecycleNotifier(), and 
> > WorkerGlobalScope::dispose() calls it. 
> > > Before this change, LifecycleObserver::contextDestroyed was called when a 
> > > WorkerGlobalScope is destructed. After this change, it is called when a 
> > > WorkerGlobalScope::dispose is called.
> > 
> > I'd like to understand why it's safe to make this change.
> 
> According to try bot result, the change was not safe.  I have removed it from
> Patch Set 2.

Thanks, LGTM.

[tkent, 2014-05-31 15:39:18]

The CQ bit was checked by tkent@chromium.org

[commit-bot: I haz the power, 2014-05-31 15:39:32]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tkent@chromium.org/304353004/60001

[commit-bot: I haz the power, 2014-05-31 20:21:26]

Change committed as 175201