Add context to gfx::Screen calls in support of simultaneous desktop+ash

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 95b6f9d1fc074465b9c90c76805d54667cd16d1d..9152ddc656d2a36aa35991721a93eed5115b1e5a 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -193,7 +193,8 @@ Shell::Shell(ShellDelegate* delegate)
       panel_layout_manager_(NULL),
       status_area_widget_(NULL),
       browser_context_(NULL) {
-  gfx::Screen::SetInstance(screen_);
+  gfx::Screen::SetScreenInstance(gfx::SCREEN_TYPE_NATIVE, screen_);
+  gfx::Screen::SetScreenInstance(gfx::SCREEN_TYPE_ALTERNATE, screen_);
   ui_controls::InstallUIControlsAura(internal::CreateUIControls());
 #if defined(OS_CHROMEOS)
   output_configurator_->AddObserver(output_configurator_animation_.get());
@@ -336,6 +337,11 @@ aura::RootWindow* Shell::GetActiveRootWindow() {
 }
 
 // static
+gfx::Screen* Shell::GetScreen() {
+  return gfx::Screen::GetScreenFor(GetPrimaryRootWindow());
+}
+
+// static
 Shell::RootWindowList Shell::GetAllRootWindows() {
   return Shell::GetInstance()->display_controller()->
       GetAllRootWindows();