base: Change WeakPtr to use SequenceChecker instead of ThreadChecker.

base/threading/sequenced_worker_pool.h

Index: base/threading/sequenced_worker_pool.h
diff --git a/base/threading/sequenced_worker_pool.h b/base/threading/sequenced_worker_pool.h
index 7e04b071428c838dfcec0faa647f82baf2932045..52ca0c9eefed626d721f935a7bef6cec135590d5 100644
--- a/base/threading/sequenced_worker_pool.h
+++ b/base/threading/sequenced_worker_pool.h
@@ -143,6 +143,11 @@ class BASE_EXPORT SequencedWorkerPool : public TaskRunner {
     virtual void OnDestruct() = 0;
   };
 
+  // Gets the SequencedToken of the current thread.
+  // If current thread is not a SequencedWorkerPool worker thread, this returns
+  // false and |result_token| is unmodified. |result_token| may not be NULL.
+  static bool CurrentThreadSequenceToken(SequenceToken* result_token);
+
   // When constructing a SequencedWorkerPool, there must be a
   // MessageLoop on the current thread unless you plan to deliberately
   // leak it.
@@ -159,7 +164,7 @@ class BASE_EXPORT SequencedWorkerPool : public TaskRunner {
                       TestingObserver* observer);
 
   // Returns a unique token that can be used to sequence tasks posted to
-  // PostSequencedWorkerTask(). Valid tokens are alwys nonzero.
+  // PostSequencedWorkerTask(). Valid tokens are always nonzero.
   SequenceToken GetSequenceToken();
 
   // Returns the sequence token associated with the given name. Calling this