SkThreadPool: allow for Runnables that add other Runnables to the pool.

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
 
@@ skipping 27 matching lines @@
 
  private:
     struct LinkedRunnable {
         // Unowned pointer.
         SkRunnable* fRunnable;
 
     private:
         SK_DECLARE_INTERNAL_LLIST_INTERFACE(LinkedRunnable);
     };
 
-    SkTInternalLList<LinkedRunnable>    fQueue;
-    SkCondVar                           fReady;
-    SkTDArray<SkThread*>                fThreads;
-    bool                                fDone;
+    enum State {
+        kRunning_State,  // Normal case.  We've been constructed and no one has called wait().
+        kWaiting_State,  // wait has been called, but there still might be work to do or being done.
+        kHalting_State,  // There's no work to do and no thread is busy.  All threads can shut down.
+    };
+
+    SkTInternalLList<LinkedRunnable> fQueue;
+    SkCondVar                        fReady;
+    SkTDArray<SkThread*>             fThreads;
+    State                            fState;
+    int                              fBusyThreads;
 
     static void Loop(void*);  // Static because we pass in this.
 };
 
 #endif