Ignore fullscreen windows which are behind other windows for fullscreen mode.

ash/root_window_controller_unittest.cc

Index: ash/root_window_controller_unittest.cc
diff --git a/ash/root_window_controller_unittest.cc b/ash/root_window_controller_unittest.cc
index d547a8a335099074ab4f11962e4f4da675a6a6d3..f77e3d9a88e9d0731948172c06e502b34af093d3 100644
--- a/ash/root_window_controller_unittest.cc
+++ b/ash/root_window_controller_unittest.cc
@@ -472,10 +472,9 @@ TEST_F(RootWindowControllerTest, GetTopmostFullscreenWindow) {
   w3->Activate();
   EXPECT_EQ(w2->GetNativeWindow(), controller->GetTopmostFullscreenWindow());
 
-  // Since there's only one desktop workspace, it always returns the same
-  // fullscreen window.
+  // If the topmost window is not fullscreen, it returns NULL.
   w1->Activate();
-  EXPECT_EQ(w2->GetNativeWindow(), controller->GetTopmostFullscreenWindow());
+  EXPECT_EQ(NULL, controller->GetTopmostFullscreenWindow());
 }
 
 TEST_F(RootWindowControllerTest, MultipleFullscreenWindows) {
@@ -496,10 +495,9 @@ TEST_F(RootWindowControllerTest, MultipleFullscreenWindows) {
   w3->Activate();
   EXPECT_EQ(w3->GetNativeWindow(), controller->GetTopmostFullscreenWindow());
 
-  // If the active window is not fullscreen, it still returns the topmost
-  // fullscreen window, which is the last active one.
+  // If the active window is not fullscreen, it returns NULL.
   w1->Activate();
-  EXPECT_EQ(w3->GetNativeWindow(), controller->GetTopmostFullscreenWindow());
+  EXPECT_EQ(NULL, controller->GetTopmostFullscreenWindow());
 }
 
 // Test that user session window can't be focused if user session blocked by