MacViews: Fix sending mouse exit event and releasing capture on D&D.

ui/views/cocoa/drag_drop_client_mac.mm

Index: ui/views/cocoa/drag_drop_client_mac.mm
diff --git a/ui/views/cocoa/drag_drop_client_mac.mm b/ui/views/cocoa/drag_drop_client_mac.mm
index f2b77998edb4a177aca2703b3d8690e4a464a4b8..5e75e4474ac81062eb8856e34e36eb18070d5b77 100644
--- a/ui/views/cocoa/drag_drop_client_mac.mm
+++ b/ui/views/cocoa/drag_drop_client_mac.mm
@@ -79,6 +79,8 @@ void DragDropClientMac::StartDragAndDrop(
   const ui::OSExchangeDataProviderMac& provider =
       static_cast<const ui::OSExchangeDataProviderMac&>(data.provider());
 
+  bridge_->ReleaseCapture();

[tapted, 2016/09/19 00:31:01]

Can you explain this change? Does it correspond to something that's done in Aura
that we forgot to add?

views::Widget calls this in Widget::OnMouseEvent() when it gets
ET_MOUSE_RELEASED - it seems strange to call it during StartDragAndDrop() as
well.

[snake, 2016/09/20 15:04:26]

Yes. 
No, this is not dead code, it is called on MouseUp event, but not called on D&D.

[tapted, 2016/09/21 10:07:05]

OK, if we don't fix it a different way, we'll need to add a comment here
outlining the case (3) you identified. E.g.

// Release capture before beginning the dragging session. Capture may have been
acquired on the mouseDown, but capture is not required during the dragging
session and the mouseUp that would release it will be suppressed.

[snake, 2016/09/21 12:29:52]

Done.

+
   // Synthesize an event for dragging, since we can't be sure that
   // [NSApp currentEvent] will return a valid dragging event.
   NSWindow* window = bridge_->ns_window();