Remove itself from the widget observers in destructor

ui/message_center/views/message_center_bubble.cc

Index: ui/message_center/views/message_center_bubble.cc
diff --git a/ui/message_center/views/message_center_bubble.cc b/ui/message_center/views/message_center_bubble.cc
index 62935c0c0616266841c99549a610bcf759c0fe94..e15398af19afce848ab118e930016e2bcf72b0b0 100644
--- a/ui/message_center/views/message_center_bubble.cc
+++ b/ui/message_center/views/message_center_bubble.cc
@@ -68,6 +68,7 @@ MessageCenterBubble::MessageCenterBubble(MessageCenter* message_center,
       first_item_has_no_margin_(first_item_has_no_margin) {}
 
 MessageCenterBubble::~MessageCenterBubble() {
+  bubble_view()->GetWidget()->RemoveObserver(this);

[msw, 2016/06/16 17:12:48]

Sorry, but TrayBubbleView is "specialized" (ie. not like typical bubbles), and I
don't know how it works offhand. Looking at BubbleViewDestroyed, I figured
bubble_view() may return null here, but it seems like that's called from the
TrayBubbleView dtor, so perhaps not? You might want to ask skuhne@, he probably
knows how tray bubbles work far better than myself.

[Mr4D (OOO till 08-26), 2016/06/17 23:10:08]

My work on bubbles is at least 2 years ago and I did not work on the
MessageCenter (or its bubble).

It appears that only the TrayBubbleView calls BubbleViewDestroyed in its dtor.

I am not sure about this either after glancing for about 15 minutes at it. I
would suggest to either let Mukai or xiyuan review. Mukai did possibly write
this, but he left Chrome, so I add both to the review.

[xiyuan, 2016/06/20 16:04:40]

It probably would crash since MessageCenterBubble has a different lifetime from
the bubble_view()'s widget, i.e. widget could be gone when MessageCenterBubble
is released.

What does the bug say? I don't have the access to the bug. My guess that that
OnWidgetClosing handle the case that widget closes before message bubble but not
the other way around, i.e. MessageCenterBubble goes before the bubble Widget.
Not sure how it would happen though (maybe signs out with the message center
bubble open)?

 }
 
 void MessageCenterBubble::SetSettingsVisible() {
@@ -120,7 +121,6 @@ void MessageCenterBubble::UpdateBubbleView() {
 }
 
 void MessageCenterBubble::OnWidgetClosing(views::Widget* widget) {
-  bubble_view()->GetWidget()->RemoveObserver(this);
   if (message_center_view_)
     message_center_view_->SetIsClosing(true);
 }