ash/mash: Introduce a ShelfDelegate implementation for mash.

ash/screen_util.cc

Index: ash/screen_util.cc
diff --git a/ash/screen_util.cc b/ash/screen_util.cc
index c27d896e52cb2e107fb31b7c1515aba754dc14f5..7e4017e13010b38991bf301f2b8da0749f9d6199 100644
--- a/ash/screen_util.cc
+++ b/ash/screen_util.cc
@@ -65,6 +65,16 @@ gfx::Rect ScreenUtil::GetShelfDisplayBoundsInRoot(aura::Window* window) {
     size.Scale(scale, scale);
     return gfx::Rect(gfx::ToCeiledSize(size));
   } else {
+    if (window->GetRootWindow()->bounds().IsEmpty()) {
+      // TODO(sad): This only happens when running with mustash, since the
+      // root-window here refers to the shelf Widget, which has not been
+      // sized/positioned yet. Use the bounds of the display in this case.
+      // Ideally, we would not run this code at all for mustash.
+      NOTIMPLEMENTED();
+      gfx::Display display =
+          gfx::Screen::GetScreen()->GetDisplayNearestWindow(window);
+      return gfx::Rect(display.size());
+    }
     return window->GetRootWindow()->bounds();
   }
 }