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 accafd9c41af3c9bf18ab9263072e7345be9f232..d45c0612d2d1cb1fdde84e61a646ef6b2c62b136 100644
--- a/ash/wm/window_util.cc
+++ b/ash/wm/window_util.cc
@@ -22,17 +22,26 @@ DEFINE_WINDOW_PROPERTY_KEY(bool, kOpenWindowSplitKey, false);
 namespace wm {
 
 void ActivateWindow(aura::Window* window) {
-  aura::client::GetActivationClient(Shell::GetRootWindow())->ActivateWindow(
+  DCHECK(window);
+  DCHECK(window->GetRootWindow());
+  aura::client::GetActivationClient(window->GetRootWindow())->ActivateWindow(
       window);
 }
 
 void DeactivateWindow(aura::Window* window) {
-  aura::client::GetActivationClient(Shell::GetRootWindow())->DeactivateWindow(
+  DCHECK(window);
+  DCHECK(window->GetRootWindow());
+  aura::client::GetActivationClient(window->GetRootWindow())->DeactivateWindow(
       window);
 }
 
 bool IsActiveWindow(aura::Window* window) {
-  return GetActiveWindow() == window;
+  DCHECK(window);
+  if (!window->GetRootWindow())
+    return false;
+
+  return aura::client::GetActivationClient(window->GetRootWindow())->
+      GetActiveWindow() == window;
 }
 
 aura::Window* GetActiveWindow() {