Added notion of currently active app / browser

ash/wm/window_util.cc

Index: ash/wm/window_util.cc
diff --git a/ash/wm/window_util.cc b/ash/wm/window_util.cc
index f7d7aaeffb5cae0538a8bfbc2f350ee814bad0f2..623eb50ed5cc699e9ef054f15a888befe91cb1bd 100644
--- a/ash/wm/window_util.cc
+++ b/ash/wm/window_util.cc
@@ -22,17 +22,23 @@ DEFINE_WINDOW_PROPERTY_KEY(bool, kOpenWindowSplitKey, false);
 namespace wm {
 
 void ActivateWindow(aura::Window* window) {
-  aura::client::GetActivationClient(Shell::GetRootWindow())->ActivateWindow(
+  DCHECK(window->GetRootWindow());
+  aura::client::GetActivationClient(window->GetRootWindow())->ActivateWindow(
       window);
 }
 
 void DeactivateWindow(aura::Window* window) {
-  aura::client::GetActivationClient(Shell::GetRootWindow())->DeactivateWindow(
+  DCHECK(window->GetRootWindow());
+  aura::client::GetActivationClient(window->GetRootWindow())->DeactivateWindow(
       window);
 }
 
 bool IsActiveWindow(aura::Window* window) {
-  return GetActiveWindow() == window;
+  if (!window->GetRootWindow())
+    return false;
+
+  return aura::client::GetActivationClient(window->GetRootWindow())->
+      GetActiveWindow() == window;
 }
 
 aura::Window* GetActiveWindow() {