mash: Migrate ScopedTargetRootWindow to //ash/common

ash/mus/bridge/wm_shell_mus.cc

Index: ash/mus/bridge/wm_shell_mus.cc
diff --git a/ash/mus/bridge/wm_shell_mus.cc b/ash/mus/bridge/wm_shell_mus.cc
index e13fbffd63d95fb9001c6973fbd5ea26556ea3b3..86882eadd9441fc8621f693808bcffcedb9d27e9 100644
--- a/ash/mus/bridge/wm_shell_mus.cc
+++ b/ash/mus/bridge/wm_shell_mus.cc
@@ -155,6 +155,9 @@ WmShellMus* WmShellMus::Get() {
 void WmShellMus::AddRootWindowController(
     WmRootWindowControllerMus* controller) {
   root_window_controllers_.push_back(controller);
+  // The first root window will be the initial root for new windows.
+  if (!root_window_for_new_windows())
+    set_root_window_for_new_windows(controller->GetWindow());
 }
 
 void WmShellMus::RemoveRootWindowController(
@@ -206,11 +209,6 @@ WmWindow* WmShellMus::GetRootWindowForDisplayId(int64_t display_id) {
   return GetRootWindowControllerWithDisplayId(display_id)->GetWindow();
 }
 
-WmWindow* WmShellMus::GetRootWindowForNewWindows() {
-  NOTIMPLEMENTED();
-  return root_window_controllers_[0]->GetWindow();
-}
-
 const DisplayInfo& WmShellMus::GetDisplayInfo(int64_t display_id) const {
   NOTIMPLEMENTED();
   static DisplayInfo fake_info;
@@ -358,6 +356,10 @@ bool WmShellMus::IsActivationParent(ui::Window* window) {
 void WmShellMus::OnWindowTreeFocusChanged(ui::Window* gained_focus,
                                           ui::Window* lost_focus) {
   WmWindowMus* gained_active = GetToplevelAncestor(gained_focus);
+  if (gained_active) {
+    set_root_window_for_new_windows(
+        static_cast<WmWindow*>(gained_active)->GetRootWindow());

[msw, 2016/08/02 01:04:50]

q: why is this cast needed? WmWindowMus::GetRootWindow should work.

[James Cook, 2016/08/02 16:16:04]

WmWindowMus::GetRootWindow calls the const version. Changed the pointers to
WmWindow*.

+  }
   WmWindowMus* lost_active = GetToplevelAncestor(gained_focus);
   if (gained_active == lost_active)
     return;