Add PDF compositor service

content/public/utility/utility_thread.h

Index: content/public/utility/utility_thread.h
diff --git a/content/public/utility/utility_thread.h b/content/public/utility/utility_thread.h
index c76efda6e1bda219392d8dfeb4bc6a3fe12c4b8b..4a9ece9c7638e9cbf7b9b915d9d83bcc0dfc991a 100644
--- a/content/public/utility/utility_thread.h
+++ b/content/public/utility/utility_thread.h
@@ -5,6 +5,8 @@
 #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 {
@@ -18,6 +20,8 @@ class CONTENT_EXPORT UtilityThread : virtual public ChildThread {
   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;