Clarify WeakPtr threading characteristic.

base/memory/weak_ptr.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Weak pointers are pointers to an object that do not affect its lifetime,
 // and which may be invalidated (i.e. reset to NULL) by the object, or its
 // owner, at any time, most commonly when the object is about to be deleted.
 
 // Weak pointers are useful when an object needs to be accessed safely by one
 // or more objects other than its owner, and those callers can cope with the
@@ skipping 41 matching lines @@
 // dereferenced and invalidated on the same SequencedTaskRunner otherwise
 // checking the pointer would be racey.
 //
 // To ensure correct use, the first time a WeakPtr issued by a WeakPtrFactory
 // is dereferenced, the factory and its WeakPtrs become bound to the calling
 // thread or current SequencedWorkerPool token, and cannot be dereferenced or
 // invalidated on any other task runner. Bound WeakPtrs can still be handed
 // off to other task runners, e.g. to use to post tasks back to object on the
 // bound sequence.
 //
-// Invalidating the factory's WeakPtrs un-binds it from the sequence, allowing
-// it to be passed for a different sequence to use or delete it.
+// If all WeakPtr objects are destroyed or invalidated then the factory is
+// unbound from the SequencedTaskRunner/Thread. The WeakPtrFactory may then be
+// destroyed, or new WeakPtr objects may be used, from a different sequence.
+//
+// Thus, at least one WeakPtr object must exist and have been dereferenced on
+// the correct thread to enforce that other WeakPtr objects will enforce they
+// are used on the desired thread.
 
 #ifndef BASE_MEMORY_WEAK_PTR_H_
 #define BASE_MEMORY_WEAK_PTR_H_
 
 #include "base/basictypes.h"
 #include "base/base_export.h"
 #include "base/logging.h"
 #include "base/memory/ref_counted.h"
 #include "base/sequence_checker.h"
 #include "base/template_util.h"
@@ skipping 257 matching lines @@
 //   base::WeakPtr<Derived> ptr = derived.AsWeakPtr();  // Fails.
 
 template <typename Derived>
 WeakPtr<Derived> AsWeakPtr(Derived* t) {
   return internal::SupportsWeakPtrBase::StaticAsWeakPtr<Derived>(t);
 }
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_WEAK_PTR_H_