mus: Implement interwindow drag and drop

ui/views/mus/BUILD.gn

 # Copyright 2014 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("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//services/shell/public/cpp/service.gni")
 import("//services/shell/public/service_manifest.gni")
 import("//testing/test.gni")
 import("//tools/grit/repack.gni")
 
 gypi = exec_script("//build/gypi_to_gn.py",
                    [ rebase_path("../views.gyp") ],
                    "scope",
                    [ "../views.gyp" ])
 component("mus") {
   output_name = "ui_views_mus_lib"
 
   sources = [
     "aura_init.cc",
     "aura_init.h",
     "clipboard_mus.cc",
     "clipboard_mus.h",
     "display_list.cc",
     "display_list.h",
+    "drop_target_mus.cc",
+    "drop_target_mus.h",
     "input_method_mus.cc",
     "input_method_mus.h",
     "mus_export.h",
     "native_widget_mus.cc",
     "native_widget_mus.h",
     "os_exchange_data_provider_mus.cc",
     "os_exchange_data_provider_mus.h",
     "pointer_watcher_event_router.cc",
     "pointer_watcher_event_router.h",
     "screen_mus.cc",
@@ skipping 267 matching lines @@
   type = "exe"
   name = "views_mus_unittests"
   source = "unittests_manifest.json"
 }
 
 service_manifest("interactive_ui_tests_manifest") {
   type = "exe"
   name = "views_mus_interactive_ui_tests"
   source = "interactive_ui_tests_manifest.json"
 }