Prepare mus_demo for external window mode some more

This CL brings no functionality changes, to makes the CL [1]
that will bring external window mode capabilities to Mus slightly
smaller.

In summary it:

- removes a loose include in mus_demo_external.h
- adds a setter for the WindowTreeHostMus in WindowTreeData class,
  as a protected member (to be used by WindowTreeDataExternal in [1])
  and uses it.
- Conditions WindowTreeData::IsInitialized to 'window_delegate_' rather
  than to 'window_tree_host_'. Reason: after [1], window_tree_host_ will
  be initialized earlier on, and not reflect a WindowTreeData instance
  actually be initialized anymore.

BUG=666958

[1] https://crrev.com/c/955782

Change-Id: Ifec202d2217cfeafd9a13c7811610fbe450b9937
Reviewed-on: https://chromium-review.googlesource.com/957143
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#542145}

services/ui/demo/mus_demo_external.h

@@ -9,7 +9,6 @@
 #include <vector>
 
 #include "services/ui/demo/mus_demo.h"
-#include "services/ui/public/interfaces/window_tree_host_factory.mojom.h"
 
 namespace ui {
 namespace demo {

services/ui/demo/window_tree_data.cc

@@ -70,7 +70,7 @@ void WindowTreeData::Init(
     std::unique_ptr<aura::WindowTreeHostMus> window_tree_host) {
   window_tree_host->Show();
   // Take ownership of the WTH.
-  window_tree_host_ = std::move(window_tree_host);
+  SetWindowTreeHost(std::move(window_tree_host));
 
   // Initialize the window for the bitmap.
   window_delegate_ = new aura_extra::ImageWindowDelegate();

services/ui/demo/window_tree_data.h

@@ -27,12 +27,18 @@ class WindowTreeData {
 
   // Initializes the window tree host and start drawing frames.
   void Init(std::unique_ptr<aura::WindowTreeHostMus> window_tree_host);
-  bool IsInitialized() const { return !!window_tree_host_; }
+  bool IsInitialized() const { return !!window_delegate_; }
 
   const aura::WindowTreeHostMus* WindowTreeHost() const {
     return window_tree_host_.get();
   }
 
+ protected:
+  void SetWindowTreeHost(
+      std::unique_ptr<aura::WindowTreeHostMus> window_tree_host) {
+    window_tree_host_ = std::move(window_tree_host);
+  }
+
  private:
   // Draws one frame, incrementing the rotation angle.
   void DrawFrame();