base: Re-apply WeakPtr support for SequencedWorkerPools, fixing deadlock

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 49 matching lines @@
 // Invalidating the factory's WeakPtrs un-binds it from the thread, allowing it
 // to be passed for a different thread to use or delete it.
 
 #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"
-#include "base/threading/thread_checker.h"
 
 namespace base {
 
 template <typename T> class SupportsWeakPtr;
 template <typename T> class WeakPtr;
 
 namespace internal {
 // These classes are part of the WeakPtr implementation.
 // DO NOT USE THESE CLASSES DIRECTLY YOURSELF.
 
 class BASE_EXPORT WeakReference {
  public:
   // Although Flag is bound to a specific thread, it may be deleted from another
   // via base::WeakPtr::~WeakPtr().
   class Flag : public RefCountedThreadSafe<Flag> {
    public:
     Flag();
 
     void Invalidate();
     bool IsValid() const;
 
     // Remove this when crbug.com/234964 is addressed.
-    void DetachFromThreadHack() { thread_checker_.DetachFromThread(); }
+    void DetachFromThreadHack() { sequence_checker_.DetachFromSequence(); }
 
    private:
     friend class base::RefCountedThreadSafe<Flag>;
 
     ~Flag();
 
-    ThreadChecker thread_checker_;
+    SequenceChecker sequence_checker_;
     bool is_valid_;
   };
 
   WeakReference();
   explicit WeakReference(const Flag* flag);
   ~WeakReference();
 
   bool is_valid() const;
 
  private:
@@ skipping 234 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_