[CrOS Wallpaper] Activate last available window on Wallpaper app close.

Currently, when the Wallpaper app closes, the most recently used window
is not re-activated.  This CL ensures that the most recently used
window is activated, allowing keyboard commands to work.

Change-Id: I1cc6e424f52f73e484bfb61044d321b6639516c8
Bug: 868543
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1629701
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665765}

ash/wallpaper/wallpaper_window_state_manager.cc

@@ -12,6 +12,26 @@
 
 namespace ash {
 
+namespace {
+
+// Reactivate the most recently unminimized window on the active desk so as not
+// to activate any desk that has been explicitly navigated away from, nor
+// activate any window that has been explicitly minimized.
+void ActivateMruUnminimizedWindowOnActiveDesk() {
+  MruWindowTracker::WindowList mru_windows(
+      Shell::Get()->mru_window_tracker()->BuildMruWindowList(
+          DesksMruType::kActiveDesk));
+  for (auto* window : mru_windows) {
+    if (wm::GetWindowState(window)->GetStateType() !=
+        WindowStateType::kMinimized) {
+      wm::GetWindowState(window)->Activate();
+      return;
+    }
+  }
+}
+
+}  // namespace
+
 WallpaperWindowStateManager::WallpaperWindowStateManager() = default;
 
 WallpaperWindowStateManager::~WallpaperWindowStateManager() = default;
@@ -63,6 +83,8 @@ void WallpaperWindowStateManager::RestoreMinimizedWindows(
     wm::GetWindowState(*iter)->Unminimize();
     RemoveObserverIfUnreferenced(*iter);
   }
+
+  ActivateMruUnminimizedWindowOnActiveDesk();
 }
 
 void WallpaperWindowStateManager::RemoveObserverIfUnreferenced(