Change ui cursor identifiers to an enum class.

ui/gfx/native_widget_types.h

Index: ui/gfx/native_widget_types.h
diff --git a/ui/gfx/native_widget_types.h b/ui/gfx/native_widget_types.h
index e2bb463602f8065019dae08a8797d011c5f0e4b3..517bd4030b96d1fc8b3afdd22ef39f3946610d5f 100644
--- a/ui/gfx/native_widget_types.h
+++ b/ui/gfx/native_widget_types.h
@@ -46,6 +46,7 @@ class Window;
 }
 namespace ui {
 class Cursor;
+enum class CursorType;
 class Event;
 }
 #endif  // defined(USE_AURA)
@@ -156,7 +157,7 @@ typedef void* NativeViewAccessible;
 
 // A constant value to indicate that gfx::NativeCursor refers to no cursor.
 #if defined(USE_AURA)
-const int kNullCursor = 0;
+const ui::CursorType kNullCursor = static_cast<ui::CursorType>(0);
 #else
 const gfx::NativeCursor kNullCursor = static_cast<gfx::NativeCursor>(NULL);
 #endif