Hook up Chrome's wallpaper picker for mash.

ash/desktop_background/desktop_background_controller.cc

Index: ash/desktop_background/desktop_background_controller.cc
diff --git a/ash/desktop_background/desktop_background_controller.cc b/ash/desktop_background/desktop_background_controller.cc
index a40974dcaf8174b084f175d032da0f561aa01099..15d18374548684a03ec22325f11dfcee20f926ed 100644
--- a/ash/desktop_background/desktop_background_controller.cc
+++ b/ash/desktop_background/desktop_background_controller.cc
@@ -161,19 +161,25 @@ void DesktopBackgroundController::OnRootWindowAdded(aura::Window* root_window) {
 
 // static
 gfx::Size DesktopBackgroundController::GetMaxDisplaySizeInNative() {
+  // Return an empty size for test environments where the screen is null.
+  if (!display::Screen::GetScreen())
+    return gfx::Size();
+
   int width = 0;
   int height = 0;
-  std::vector<display::Display> displays =
-      display::Screen::GetScreen()->GetAllDisplays();
-  DisplayManager* display_manager = Shell::GetInstance()->display_manager();
-
-  for (std::vector<display::Display>::iterator iter = displays.begin();
-       iter != displays.end(); ++iter) {
+  DisplayManager* display_manager =
+      Shell::HasInstance() ? Shell::GetInstance()->display_manager() : nullptr;
+  for (auto& display : display::Screen::GetScreen()->GetAllDisplays()) {
     // Don't use size_in_pixel because we want to use the native pixel size.
+    // TODO(msw): Fix this for Mash/Mus; see http://crbug.com/613657.
     gfx::Size size_in_pixel =
-        display_manager->GetDisplayInfo(iter->id()).bounds_in_native().size();
-    if (iter->rotation() == display::Display::ROTATE_90 ||
-        iter->rotation() == display::Display::ROTATE_270) {
+        display_manager
+            ? display_manager->GetDisplayInfo(display.id())
+                  .bounds_in_native()
+                  .size()
+            : display.size();
+    if (display.rotation() == display::Display::ROTATE_90 ||
+        display.rotation() == display::Display::ROTATE_270) {
       size_in_pixel = gfx::Size(size_in_pixel.height(), size_in_pixel.width());
     }
     width = std::max(size_in_pixel.width(), width);