Aura: Don't activate menus and other non-activatable widgets on drop

BUG=none

Review URL: https://codereview.chromium.org/517103003

Cr-Commit-Position: refs/heads/master@{#292848}

ui/views/widget/desktop_aura/desktop_native_widget_aura.cc

@@ -1140,7 +1140,8 @@ void DesktopNativeWidgetAura::OnDragExited() {
 
 int DesktopNativeWidgetAura::OnPerformDrop(const ui::DropTargetEvent& event) {
   DCHECK(drop_helper_.get() != NULL);
-  Activate();
+  if (ShouldActivate())
+    Activate();
   return drop_helper_->OnDrop(event.data(), event.location(),
       last_drop_operation_);
 }