Implement "pinned" mode in ash.

ash/mus/bridge/wm_shell_mus.h

Index: ash/mus/bridge/wm_shell_mus.h
diff --git a/ash/mus/bridge/wm_shell_mus.h b/ash/mus/bridge/wm_shell_mus.h
index 39859515743c59b60f4619fc4b81719103fb11ac..86e87c280c87eeeaf34b12ee41f6224102c20ca4 100644
--- a/ash/mus/bridge/wm_shell_mus.h
+++ b/ash/mus/bridge/wm_shell_mus.h
@@ -53,6 +53,8 @@ class WmShellMus : public WmShell, public ::mus::WindowTreeClientObserver {
   WmWindow* GetRootWindowForDisplayId(int64_t display_id) override;
   WmWindow* GetRootWindowForNewWindows() override;
   bool IsForceMaximizeOnFirstRun() override;
+  bool IsPinned() override;
+  void SetPinnedWindow(WmWindow* window) override;
   bool CanShowWindowForUser(WmWindow* window) override;
   void LockCursor() override;
   void UnlockCursor() override;
@@ -73,7 +75,6 @@ class WmShellMus : public WmShell, public ::mus::WindowTreeClientObserver {
   void RemoveDisplayObserver(WmDisplayObserver* observer) override;
   void AddShellObserver(ShellObserver* observer) override;
   void RemoveShellObserver(ShellObserver* observer) override;
-  void NotifyPinnedStateChanged(WmWindow* pinned_window) override;
 
  private:
   // Returns true if |window| is a window that can have active children.