Add thread-per-core setting to SkThreadPool.

include/utils/SkThreadPool.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkThreadPool_DEFINED
 #define SkThreadPool_DEFINED
 
 #include "SkCondVar.h"
 #include "SkTDArray.h"
 #include "SkTInternalLList.h"
 
 class SkRunnable;
 class SkThread;
 
 class SkThreadPool {
 
 public:
     /**
-     * Create a threadpool with exactly count (>=0) threads.
+     * Create a threadpool with count threads, or one thread per core if kThreadPerCore.
      */
+    static const int kThreadPerCore = -1;
     explicit SkThreadPool(int count);
     ~SkThreadPool();
 
     /**
      * Queues up an SkRunnable to run when a thread is available, or immediately if
      * count is 0.  NULL is a safe no-op.  Does not take ownership.
      */
     void add(SkRunnable*);
 
  private:
     struct LinkedRunnable {
         // Unowned pointer.
         SkRunnable* fRunnable;
 
     private:
         SK_DECLARE_INTERNAL_LLIST_INTERFACE(LinkedRunnable);
     };
 
     SkTInternalLList<LinkedRunnable>    fQueue;
     SkCondVar                           fReady;
     SkTDArray<SkThread*>                fThreads;
     bool                            fDone;
 
     static void Loop(void*);  // Static because we pass in this.
 };
 
 #endif