Clarify WeakPtr threading characteristic.

base/memory/weak_ptr.h

Index: base/memory/weak_ptr.h
diff --git a/base/memory/weak_ptr.h b/base/memory/weak_ptr.h
index 1230ead14bf76513cfbc63e22a855db35ac91787..c335ae9852bdcf69b55563c217bb16acebc24fdb 100644
--- a/base/memory/weak_ptr.h
+++ b/base/memory/weak_ptr.h
@@ -59,8 +59,13 @@
 // 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_