aura-mus: wire up drag and drop.

ui/views/mus/desktop_window_tree_host_mus.cc

 // 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.
 
 #include "ui/views/mus/desktop_window_tree_host_mus.h"
 
 #include "base/memory/ptr_util.h"
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/threading/thread_task_runner_handle.h"
@@ skipping 289 matching lines @@
   // which is owned by the window manager. This way the window manager can
   // properly identify and honor transients.
   if (params.parent && params.parent->GetHost()) {
     aura::client::GetTransientWindowClient()->AddTransientChild(
         params.parent->GetHost()->window(), window());
   }
 
   if (!params.accept_events) {
     aura::WindowPortMus::Get(window())->SetEventTargetingPolicy(
         ui::mojom::EventTargetingPolicy::NONE);
+  } else {
+    // Only enable drops if the window accepts events.

[sky, 2017/03/10 21:00:57]

Did you mean to have a TODO here?

[Elliot Glaysher, 2017/03/10 21:30:07]

No, this comment is descriptive that we only want to opt in for drags if we
otherwise accept events.

I killed it since it seemed to confuse you, though.

+    aura::WindowPortMus::Get(content_window)->SetCanAcceptDrops(true);
   }
 }
 
 void DesktopWindowTreeHostMus::OnNativeWidgetCreated(
     const Widget::InitParams& params) {
   window()->SetName(params.name);
   desktop_native_widget_aura_->content_window()->SetName(
       "DesktopNativeWidgetAura - content window");
   if (params.parent && params.parent->GetHost()) {
     parent_ = static_cast<DesktopWindowTreeHostMus*>(params.parent->GetHost());
@@ skipping 493 matching lines @@
     aura::client::FocusClient* focus_client,
     aura::Window* window) {
   if (window == this->window()) {
     desktop_native_widget_aura_->HandleActivationChanged(true);
   } else if (is_active_) {
     desktop_native_widget_aura_->HandleActivationChanged(false);
   }
 }
 
 }  // namespace views