aura-mus: wire up drag and drop.

ui/views/mus/desktop_window_tree_host_mus.cc

Index: ui/views/mus/desktop_window_tree_host_mus.cc
diff --git a/ui/views/mus/desktop_window_tree_host_mus.cc b/ui/views/mus/desktop_window_tree_host_mus.cc
index 6efc8ffb2e1642b1ae3a9ce73c8e1705061c076f..e0ed950fad4b1154f269d422431f938b2cc23283 100644
--- a/ui/views/mus/desktop_window_tree_host_mus.cc
+++ b/ui/views/mus/desktop_window_tree_host_mus.cc
@@ -307,6 +307,9 @@ void DesktopWindowTreeHostMus::Init(aura::Window* content_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);
   }
 }