Add thread-per-core setting to SkThreadPool.

include/utils/SkThreadPool.h

Index: include/utils/SkThreadPool.h
diff --git a/include/utils/SkThreadPool.h b/include/utils/SkThreadPool.h
index cc45fc2cd53f9f2317c7301ad25bd9b181bb0fbb..3c8615809089a21d1076c62642f3ba4d9b218cd3 100644
--- a/include/utils/SkThreadPool.h
+++ b/include/utils/SkThreadPool.h
@@ -19,8 +19,9 @@ 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();