Add PDF compositor service

content/public/utility/utility_thread.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_PUBLIC_UTILITY_UTILITY_THREAD_H_
 #define CONTENT_PUBLIC_UTILITY_UTILITY_THREAD_H_
 
+#include "base/memory/ref_counted.h"
+#include "base/single_thread_task_runner.h"
 #include "content/public/child/child_thread.h"
 
 namespace content {
 
 class CONTENT_EXPORT UtilityThread : virtual public ChildThread {
  public:
   // Returns the one utility thread for this process.  Note that this can only
   // be accessed when running on the utility thread itself.
   static UtilityThread* Get();
 
   UtilityThread();
   ~UtilityThread() override;
 
+  virtual scoped_refptr<base::SingleThreadTaskRunner> GetIOTaskRunner() = 0;

[jam, 2017/04/28 15:49:10]

since RenderThread (another class derived from ChildThread) also has this, can
you promote this method to ChildThread and remove the declaration from here and
RenderThread?

[Wei Li, 2017/04/29 04:35:18]

Good idea. I moved it to ChildThread, but there is a subtle difference b/w the
implementation of RenderThreadImpl and ChildThreadImpl, so still leave them
there. Ideally, we can figure out why the difference and merge into one. 

+
   // Releases the process if we are not (or no longer) in batch mode.
   virtual void ReleaseProcessIfNeeded() = 0;
 
   // Initializes blink if it hasn't already been initialized.
   virtual void EnsureBlinkInitialized() = 0;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_UTILITY_UTILITY_THREAD_H_