Moves DragDropClient to aura/client

I'm going to need this in aura/client for mus.

BUG=659155
TEST=covered by tests
R=ben@chromium.org

Review-Url: https://codereview.chromium.org/2461493002
Cr-Commit-Position: refs/heads/master@{#428121}

diff --git a/ui/aura/BUILD.gn b/ui/aura/BUILD.gn
index 5862a402..e6db04b 100644
--- a/ui/aura/BUILD.gn
+++ b/ui/aura/BUILD.gn

@@ -15,8 +15,6 @@
     "../wm/public/activation_delegate.h",
     "../wm/public/animation_host.cc",
     "../wm/public/animation_host.h",
-    "../wm/public/drag_drop_client.cc",
-    "../wm/public/drag_drop_client.h",
     "../wm/public/drag_drop_delegate.cc",
     "../wm/public/drag_drop_delegate.h",
     "../wm/public/scoped_drag_drop_disabler.cc",
@@ -40,6 +38,8 @@
     "client/cursor_client_observer.h",
     "client/default_capture_client.cc",
     "client/default_capture_client.h",
+    "client/drag_drop_client.cc",
+    "client/drag_drop_client.h",
     "client/event_client.cc",
     "client/event_client.h",
     "client/focus_change_observer.cc",