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

base/sequence_checker.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.
 
 #ifndef BASE_SEQUENCE_CHECKER_H_
 #define BASE_SEQUENCE_CHECKER_H_
 
 #include "base/memory/ref_counted.h"
 
 // See comments for the similar block in thread_checker.h.
@@ skipping 10 matching lines @@
 namespace base {
 
 class SequencedTaskRunner;
 
 // Do nothing implementation, for use in release mode.
 //
 // Note: You should almost always use the SequenceChecker class to get
 // the right version for your build configuration.
 class SequenceCheckerDoNothing {
  public:
-  bool CalledOnValidSequence() const {
+  bool CalledOnValidSequencedThread() const {
     return true;
   }
 
-  void ChangeSequence(
-      const scoped_refptr<SequencedTaskRunner>& sequenced_task_runner) {}
+  void DetachFromSequence() {}
 };
 
 // SequenceChecker is a helper class used to help verify that some
 // methods of a class are called in sequence -- that is, called from
 // the same SequencedTaskRunner. It is a generalization of
 // ThreadChecker; see comments in sequence_checker_impl.h for details.
 //
 // Example:
 // class MyClass {
 //  public:
-//   explicit MyClass(
-//       const scoped_refptr<SequencedTaskRunner>& sequenced_task_runner)
-//       : sequence_checker_(sequenced_task_runner) {}
-//
 //   void Foo() {
 //     DCHECK(sequence_checker_.CalledOnValidSequence());
 //     ... (do stuff) ...
 //   }
 //
 //  private:
 //   SequenceChecker sequence_checker_;
 // }
 //
 // In Release mode, CalledOnValidSequence will always return true.
 #if ENABLE_SEQUENCE_CHECKER
 class SequenceChecker : public SequenceCheckerImpl {
- public:
-  explicit SequenceChecker(
-      const scoped_refptr<SequencedTaskRunner>& sequenced_task_runner)
-      : SequenceCheckerImpl(sequenced_task_runner) {}
 };
 #else
 class SequenceChecker : public SequenceCheckerDoNothing {
- public:
-  explicit SequenceChecker(
-      const scoped_refptr<SequencedTaskRunner>& sequenced_task_runner) {}
 };
 #endif  // ENABLE_SEQUENCE_CHECKER
 
 #undef ENABLE_SEQUENCE_CHECKER
 
 }  // namespace base
 
 #endif  // BASE_SEQUENCE_CHECKER_H_