ash/mash: Add a mus-client that sets up ash to provide the system ui for mash.

ash/host/ash_window_tree_host.cc

Index: ash/host/ash_window_tree_host.cc
diff --git a/ash/host/ash_window_tree_host.cc b/ash/host/ash_window_tree_host.cc
index 5cbe067e6ec2fd13b38c13a3f361afaabfbebeeb..0bc59558f36effcf81d2c04348067faf9a6147e0 100644
--- a/ash/host/ash_window_tree_host.cc
+++ b/ash/host/ash_window_tree_host.cc
@@ -21,6 +21,12 @@
 
 namespace ash {
 
+namespace {
+
+AshWindowTreeHost::Factory creation_factory;
+
+}  // namespace
+
 AshWindowTreeHost::AshWindowTreeHost() : input_method_handler_(nullptr) {
 }
 
@@ -49,8 +55,12 @@ void AshWindowTreeHost::TranslateLocatedEvent(ui::LocatedEvent* event) {
   }
 }
 
+// static
 AshWindowTreeHost* AshWindowTreeHost::Create(
     const AshWindowTreeHostInitParams& init_params) {
+  if (!creation_factory.is_null())
+    return creation_factory.Run(init_params);
+
   if (init_params.offscreen)
     return new AshWindowTreeHostUnified(init_params.initial_bounds);
 #if defined(USE_OZONE)
@@ -64,4 +74,9 @@ AshWindowTreeHost* AshWindowTreeHost::Create(
 #endif
 }
 
+// static
+void AshWindowTreeHost::SetFactory(const Factory& factory) {
+  creation_factory = factory;
+}
+
 }  // namespace ash