Fix MaximizeModeWIndowManagerTest with jessie case.

ash/wm/screen_pinning_controller.cc

Index: ash/wm/screen_pinning_controller.cc
diff --git a/ash/wm/screen_pinning_controller.cc b/ash/wm/screen_pinning_controller.cc
index 4a708dbf21d3cdd59888858d18a454c806a0ac4a..d3f9aa1cf4698bc30fdaa5eb9d06e58c576531f4 100644
--- a/ash/wm/screen_pinning_controller.cc
+++ b/ash/wm/screen_pinning_controller.cc
@@ -171,8 +171,6 @@ bool ScreenPinningController::IsPinned() const {
 }
 
 void ScreenPinningController::SetPinnedWindow(WmWindow* pinned_window) {
-  window_dimmers_->clear();
-
   if (pinned_window->GetWindowState()->IsPinned()) {
     if (pinned_window_) {
       LOG(DFATAL) << "Pinned mode is enabled, while it is already in "
@@ -228,6 +226,7 @@ void ScreenPinningController::SetPinnedWindow(WmWindow* pinned_window) {
                                pinned_container_child_window_observer_.get());
     WmWindow::GetAuraWindow(container)->RemoveObserver(
         pinned_container_window_observer_.get());
+    window_dimmers_->clear();
 
     pinned_window_ = nullptr;
   }