Remove now-unnecessary wrappers around gfx::Screen::GetScreen()

ash/shell/content/client/shell_browser_main_parts.cc

Index: ash/shell/content/client/shell_browser_main_parts.cc
diff --git a/ash/shell/content/client/shell_browser_main_parts.cc b/ash/shell/content/client/shell_browser_main_parts.cc
index 2a9613b29268f9b4af7ba0f79ac171b1f88db7a5..73bde5516793b437b4bcc076c5acf145a2dffb30 100644
--- a/ash/shell/content/client/shell_browser_main_parts.cc
+++ b/ash/shell/content/client/shell_browser_main_parts.cc
@@ -134,8 +134,7 @@ void ShellBrowserMainParts::PreMainMessageLoopRun() {
   ash::Shell::GetInstance()->UpdateAfterLoginStatusChange(user::LOGGED_IN_USER);
 
   window_watcher_.reset(new ash::shell::WindowWatcher);
-  gfx::Screen* screen = Shell::GetInstance()->GetScreen();
-  screen->AddObserver(window_watcher_.get());
+  gfx::Screen::GetScreen()->AddObserver(window_watcher_.get());
 
   ash::shell::InitWindowTypeLauncher();
 
@@ -143,8 +142,7 @@ void ShellBrowserMainParts::PreMainMessageLoopRun() {
 }
 
 void ShellBrowserMainParts::PostMainMessageLoopRun() {
-  gfx::Screen* screen = Shell::GetInstance()->GetScreen();
-  screen->RemoveObserver(window_watcher_.get());
+  gfx::Screen::GetScreen()->RemoveObserver(window_watcher_.get());
 
   window_watcher_.reset();
   delegate_ = nullptr;