Fix MaximizeModeWIndowManagerTest with jessie case.

ash/wm/screen_pinning_controller.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/wm/screen_pinning_controller.h"
 
 #include <algorithm>
 #include <vector>
 
 #include "ash/common/window_user_data.h"
@@ skipping 153 matching lines @@
 
 ScreenPinningController::~ScreenPinningController() {
   window_tree_host_manager_->RemoveObserver(this);
 }
 
 bool ScreenPinningController::IsPinned() const {
   return pinned_window_ != nullptr;
 }
 
 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 "
                   << "the pinned mode";
       return;
     }
 
     WmWindow* container = pinned_window->GetParent();
     std::vector<WmWindow*> system_modal_containers =
         GetSystemModalWindowsExceptPinned(pinned_window);
@@ skipping 35 matching lines @@
          GetSystemModalWindowsExceptPinned(pinned_window_)) {
       RemoveObserverFromChildren(
           system_modal, system_modal_container_child_window_observer_.get());
       WmWindow::GetAuraWindow(system_modal)
           ->RemoveObserver(system_modal_container_window_observer_.get());
     }
     RemoveObserverFromChildren(container,
                                pinned_container_child_window_observer_.get());
     WmWindow::GetAuraWindow(container)->RemoveObserver(
         pinned_container_window_observer_.get());
+    window_dimmers_->clear();
 
     pinned_window_ = nullptr;
   }
 
   Shell::GetInstance()->NotifyPinnedStateChanged(pinned_window);
 }
 
 void ScreenPinningController::OnWindowAddedToPinnedContainer(
     WmWindow* new_window) {
   KeepPinnedWindowOnTop();
@@ skipping 105 matching lines @@
     return;
 
   WindowDimmer* window_dimmer = window_dimmers_->Get(container);
   if (window_dimmer) {
     base::AutoReset<bool> auto_reset(&in_restacking_, true);
     container->StackChildAtBottom(window_dimmer->window());
   }
 }
 
 }  // namespace ash