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

content/browser/gpu/gpu_process_host.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 CONTENT_BROWSER_GPU_GPU_PROCESS_HOST_H_
 #define CONTENT_BROWSER_GPU_GPU_PROCESS_HOST_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 16 matching lines @@
 #include "gpu/config/gpu_info.h"
 #include "gpu/ipc/common/surface_handle.h"
 #include "ipc/ipc_sender.h"
 #include "ipc/message_filter.h"
 #include "ui/gfx/geometry/size.h"
 #include "ui/gfx/gpu_memory_buffer.h"
 #include "url/gurl.h"
 
 struct GPUCreateCommandBufferConfig;
 
+namespace base {
+class Thread;
+}
+
 namespace IPC {
 struct ChannelHandle;
 }
 
 namespace gpu {
 struct GpuPreferences;
 struct SyncToken;
 }
 
 namespace shell {
@@ skipping 240 matching lines @@
   ClientIdToShaderCacheMap client_id_to_shader_cache_;
 
   std::string shader_prefix_key_;
 
   DISALLOW_COPY_AND_ASSIGN(GpuProcessHost);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_GPU_GPU_PROCESS_HOST_H_