Remove dependency of SSLPlatformKeyTaskRunner on single-threaded SequencedWorkerPool.

net/ssl/threaded_ssl_private_key.h

 // Copyright 2015 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 NET_SSL_THREADED_SSL_PRIVATE_KEY_H_
 #define NET_SSL_THREADED_SSL_PRIVATE_KEY_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 34 matching lines @@
     // error code. It will only be called on the task runner passed to the
     // owning ThreadedSSLPrivateKey.
     virtual Error SignDigest(Hash hash,
                              const base::StringPiece& input,
                              std::vector<uint8_t>* signature) = 0;
 
    private:
     DISALLOW_COPY_AND_ASSIGN(Delegate);
   };
 
-  ThreadedSSLPrivateKey(std::unique_ptr<Delegate> delegate,
-                        scoped_refptr<base::TaskRunner> task_runner);
+  ThreadedSSLPrivateKey(
+      std::unique_ptr<Delegate> delegate,
+      scoped_refptr<base::SingleThreadTaskRunner> task_runner);
 
   // SSLPrivateKey implementation.
   Type GetType() override;
   std::vector<SSLPrivateKey::Hash> GetDigestPreferences() override;
   size_t GetMaxSignatureLengthInBytes() override;
   void SignDigest(Hash hash,
                   const base::StringPiece& input,
                   const SignCallback& callback) override;
 
  private:
   ~ThreadedSSLPrivateKey() override;
   class Core;
 
   scoped_refptr<Core> core_;
-  scoped_refptr<base::TaskRunner> task_runner_;
+  scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
   base::WeakPtrFactory<ThreadedSSLPrivateKey> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ThreadedSSLPrivateKey);
 };
 
 }  // namespace net
 
 #endif  // NET_SSL_THREADED_SSL_PRIVATE_KEY_H_