Add a DCHECK to ensure that WindowTreeClient has a delegate

ui/aura/mus/window_tree_client.cc

Index: ui/aura/mus/window_tree_client.cc
diff --git a/ui/aura/mus/window_tree_client.cc b/ui/aura/mus/window_tree_client.cc
index 9b2079e04c3f9815082525dd3e330329c8c5f627..5aaacb352cc0afb0b638543dd3eff4aee23c8792 100644
--- a/ui/aura/mus/window_tree_client.cc
+++ b/ui/aura/mus/window_tree_client.cc
@@ -178,6 +178,7 @@ WindowTreeClient::WindowTreeClient(
       tree_(nullptr),
       in_destructor_(false),
       weak_factory_(this) {
+  DCHECK(delegate_);

[tonikitoo, 2017/02/17 17:10:28]

alternatively, we could make 'delegate_' a reference instead, but it would be a
bigger CL.

   // Allow for a null request in tests.
   if (request.is_pending())
     binding_.Bind(std::move(request));