Removes ash::mus::RootWindowController

The functionality is moved into two places:
. Maintaining the display goes to ScreenMus.
. Creating the window moves to top_level_window_factory.

As part of this I'm removing some functionality that staggered top
level windows. I suspect this isn't used much and needs some
changes. Will address this later.

BUG=671246
TEST=none
[email protected]

Review-Url: https://codereview.chromium.org/2642003002
Cr-Commit-Position: refs/heads/master@{#444745}
diff --git a/ash/root_window_controller.h b/ash/root_window_controller.h
index 56610f5..69b0c13 100644
--- a/ash/root_window_controller.h
+++ b/ash/root_window_controller.h
@@ -67,7 +67,7 @@
 class WorkspaceController;
 
 namespace mus {
-class RootWindowController;
+class WindowManager;
 }
 
 namespace wm {
@@ -291,7 +291,7 @@
  private:
   // TODO(sky): remove this. Temporary during ash-mus unification.
   // http://crbug.com/671246.
-  friend class mus::RootWindowController;
+  friend class mus::WindowManager;
 
   // Creates a new RootWindowController with the specified host. Only one of
   // |ash_host| or |window_tree_host| should be specified. This takes ownership