Remove dependency of SSLPlatformKeyTaskRunner on single-threaded SequencedWorkerPool.

net/ssl/threaded_ssl_private_key.cc

Index: net/ssl/threaded_ssl_private_key.cc
diff --git a/net/ssl/threaded_ssl_private_key.cc b/net/ssl/threaded_ssl_private_key.cc
index a1294fadc4ce762fc2c009add67ff46ea177e945..42f01f6c0ed315916e4ec09cab49fbcca3b896b9 100644
--- a/net/ssl/threaded_ssl_private_key.cc
+++ b/net/ssl/threaded_ssl_private_key.cc
@@ -9,7 +9,7 @@
 
 #include "base/bind.h"
 #include "base/location.h"
-#include "base/task_runner.h"
+#include "base/single_thread_task_runner.h"
 #include "base/task_runner_util.h"
 #include "base/threading/thread_task_runner_handle.h"
 
@@ -50,7 +50,7 @@ class ThreadedSSLPrivateKey::Core
 
 ThreadedSSLPrivateKey::ThreadedSSLPrivateKey(
     std::unique_ptr<ThreadedSSLPrivateKey::Delegate> delegate,
-    scoped_refptr<base::TaskRunner> task_runner)
+    scoped_refptr<base::SingleThreadTaskRunner> task_runner)
     : core_(new Core(std::move(delegate))),
       task_runner_(std::move(task_runner)),
       weak_factory_(this) {}