Reland change for aura tooltips that was revert due to build break:

ui/aura/client/aura_constants.h

Index: ui/aura/client/aura_constants.h
diff --git a/ui/aura/client/aura_constants.h b/ui/aura/client/aura_constants.h
index 363c42037b070b6a3ce9d9d024692db5dcd445d4..dbc321dc4c3e719101b78da4510b5097681f5137 100644
--- a/ui/aura/client/aura_constants.h
+++ b/ui/aura/client/aura_constants.h
@@ -40,6 +40,10 @@ AURA_EXPORT extern const char kDesktopDragDropClientKey[];
 // A property key to store the drag and drop delegate for a window. The type of
 // the value is |aura::WindowDragDropDelegate*|.
 AURA_EXPORT extern const char kDragDropDelegateKey[];
+
+// A property key to store the tooltip client for the desktop. The type of
+// the value is |aura::TooltipClient*|.
+AURA_EXPORT extern const char kDesktopTooltipClientKey[];
 }  // namespace aura
 
 #endif  // UI_AURA_CLIENT_AURA_CONSTANTS_H_