Extract shell methods from ApplicationImpl into a base class, and pass this to Initialize() instead.

components/mus/ws/window_tree_host_impl.cc

Index: components/mus/ws/window_tree_host_impl.cc
diff --git a/components/mus/ws/window_tree_host_impl.cc b/components/mus/ws/window_tree_host_impl.cc
index 885750a31551ce140c37a8d23bcb41b75ba6e5b9..d75b184a806be277716d0b7fc5c9ce2d45450b3b 100644
--- a/components/mus/ws/window_tree_host_impl.cc
+++ b/components/mus/ws/window_tree_host_impl.cc
@@ -87,7 +87,7 @@ WindowTreeHostImpl::QueuedEvent::~QueuedEvent() {}
 
 WindowTreeHostImpl::WindowTreeHostImpl(
     ConnectionManager* connection_manager,
-    mojo::ApplicationImpl* app_impl,
+    mojo::Shell* shell,
     const scoped_refptr<GpuState>& gpu_state,
     const scoped_refptr<SurfacesState>& surfaces_state)
     : id_(next_id++),
@@ -95,7 +95,7 @@ WindowTreeHostImpl::WindowTreeHostImpl(
       connection_manager_(connection_manager),
       event_dispatcher_(this),
       display_manager_(
-          DisplayManager::Create(app_impl, gpu_state, surfaces_state)),
+          DisplayManager::Create(shell, gpu_state, surfaces_state)),
       tree_awaiting_input_ack_(nullptr),
       last_cursor_(0) {
   frame_decoration_values_ = mojom::FrameDecorationValues::New();