MenuRunner: Add comment to specify blocking behavior on MacViews.

ui/views/controls/menu/menu_runner.h

Index: ui/views/controls/menu/menu_runner.h
diff --git a/ui/views/controls/menu/menu_runner.h b/ui/views/controls/menu/menu_runner.h
index f0adccf8e9d96e4846e1ae1ddcc5d2337aba51f8..e2518da30ac8ebd0e3ee60eb3a4dad93f5b4fb2c 100644
--- a/ui/views/controls/menu/menu_runner.h
+++ b/ui/views/controls/menu/menu_runner.h
@@ -79,11 +79,14 @@ class VIEWS_EXPORT MenuRunner {
     FOR_DROP = 1 << 2,
 
     // The menu is a context menu (not necessarily nested), for example right
-    // click on a link on a website in the browser.
+    // click on a link on a website in the browser. On Mac(Views), this is
+    // displayed using a native NSMenu and hence is blocking.
     CONTEXT_MENU = 1 << 3,
 
-    // The menu should behave like a Windows native Combobox dropdow menu.
-    // This behavior includes accepting the pending item and closing on F4.
+    // The menu should behave like a Windows native Combobox dropdow menu. This
+    // behavior includes accepting the pending item and closing on F4. On
+    // Mac(Views), this is displayed using a native NSMenu and hence is
+    // blocking.
     COMBOBOX = 1 << 4,
 
     // A child view is performing a drag-and-drop operation, so the menu should
@@ -93,7 +96,8 @@ class VIEWS_EXPORT MenuRunner {
     NESTED_DRAG = 1 << 5,
 
     // Used for showing a menu which does NOT block the caller. Instead the
-    // delegate is notified when the menu closes via OnMenuClosed.
+    // delegate is notified when the menu closes via OnMenuClosed. On
+    // Mac(Views), this is ignored for the CONTEXT_MENU and COMBOBOX run types.
     ASYNC = 1 << 6,
   };