Apply new MD shadows to CrOS tray bubbles.

ui/wm/core/shadow.h

Index: ui/wm/core/shadow.h
diff --git a/ui/wm/core/shadow.h b/ui/wm/core/shadow.h
index e0b7a1db301c88aa440c6d48f9091a6ef4348a72..40aaa4e9c69a8cba9a0e5b252f1ace09ee50f7a2 100644
--- a/ui/wm/core/shadow.h
+++ b/ui/wm/core/shadow.h
@@ -21,6 +21,7 @@ namespace wm {
 // Simple class that draws a drop shadow around content at given bounds.
 class WM_EXPORT Shadow : public ui::ImplicitAnimationObserver {
  public:
+  // TODO(estade): remove this enum and instead set elevation directly.
   enum Style {
     // Active windows have more opaque shadows, shifted down to make the window
     // appear "higher".