Remove unused thread.h include in trace_event_impl.h and fix IWUU.

components/leveldb/leveldb_mojo_proxy.h

 // Copyright 2016 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 COMPONENTS_LEVELDB_LEVELDB_MOJO_PROXY_H_
 #define COMPONENTS_LEVELDB_LEVELDB_MOJO_PROXY_H_
 
 #include <map>
 #include <memory>
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "base/callback_forward.h"
 #include "base/files/file.h"
 #include "base/memory/ref_counted.h"
 #include "base/synchronization/waitable_event.h"
-#include "base/threading/thread.h"
 #include "components/filesystem/public/interfaces/directory.mojom.h"
 
+namespace base {
+class SingleThreadTaskRunner;
+}
+
 namespace leveldb {
 
 // A proxy for thread safe access to Mojo objects from multiple threads.
 //
 // MojoEnv is an object passed to the LevelDB implementation which can be
 // called from multiple threads. Mojo pipes are bound to a single
 // thread. Because of this mismatch, we create a proxy object which will
 // redirect calls to the thread which owns the Mojo pipe, sends and receives
 // messages.
 //
@@ skipping 123 matching lines @@
   scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
 
   int outstanding_opaque_dirs_;
 
   DISALLOW_COPY_AND_ASSIGN(LevelDBMojoProxy);
 };
 
 }  // namespace leveldb
 
 #endif  // COMPONENTS_LEVELDB_LEVELDB_MOJO_PROXY_H_