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

ui/views/cocoa/drag_drop_client_mac.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ui/views/cocoa/drag_drop_client_mac.h"
 
 #include "base/mac/mac_util.h"
 #include "base/run_loop.h"
 #include "base/strings/sys_string_conversions.h"
 #import "ui/base/dragdrop/os_exchange_data_provider_mac.h"
@@ skipping 61 matching lines @@
     View* view,
     const ui::OSExchangeData& data,
     int operation,
     ui::DragDropTypes::DragEventSource source) {
   data_source_.reset([[CocoaDragDropDataProvider alloc] initWithData:data]);
   operation_ = operation;
 
   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();
   NSPoint position = [window mouseLocationOutsideOfEventStream];
   NSTimeInterval event_time = [[NSApp currentEvent] timestamp];
   NSEvent* event = [NSEvent mouseEventWithType:NSLeftMouseDragged
                                       location:position
                                  modifierFlags:NSLeftMouseDraggedMask
                                      timestamp:event_time
                                   windowNumber:[window windowNumber]
@@ skipping 67 matching lines @@
     quit_closure_.Run();
     quit_closure_.Reset();
   }
 }
 
 gfx::Point DragDropClientMac::LocationInView(NSPoint point) const {
   return gfx::Point(point.x, NSHeight([bridge_->ns_window() frame]) - point.y);
 }
 
 }  // namespace views