Mojo: Delete some old, unused code.

mojo/system/handle_table.h

Index: mojo/system/handle_table.h
diff --git a/mojo/system/handle_table.h b/mojo/system/handle_table.h
index 96f7abae7450353316440ecd1b3bf325df74f92f..1ccc96433700b1898053a478df870efc47831b5e 100644
--- a/mojo/system/handle_table.h
+++ b/mojo/system/handle_table.h
@@ -101,6 +101,23 @@ class MOJO_SYSTEM_IMPL_EXPORT HandleTable {
  private:
   friend bool internal::ShutdownCheckNoLeaks(Core*);
 
+  // The |busy| member is used only to deal with functions (in particular
+  // |Core::WriteMessage()|) that want to hold on to a dispatcher and later
+  // remove it from the handle table, without holding on to the handle table
+  // lock.
+  //
+  // For example, if |Core::WriteMessage()| is called with a handle to be sent,
+  // (under the handle table lock) it must first check that that handle is not
+  // busy (if it is busy, then it fails with |MOJO_RESULT_BUSY|) and then marks
+  // it as busy. To avoid deadlock, it should also try to acquire the locks for
+  // all the dispatchers for the handles that it is sending (and fail with
+  // |MOJO_RESULT_BUSY| if the attempt fails). At this point, it can release the
+  // handle table lock.
+  //
+  // If |Core::Close()| is simultaneously called on that handle, it too checks
+  // if the handle is marked busy. If it is, it fails (with |MOJO_RESULT_BUSY|).
+  // This prevents |Core::WriteMessage()| from sending a handle that has been
+  // closed (or learning about this too late).
   struct Entry {
     Entry();
     explicit Entry(const scoped_refptr<Dispatcher>& dispatcher);