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

include/utils/SkThreadPool.h

Index: include/utils/SkThreadPool.h
diff --git a/include/utils/SkThreadPool.h b/include/utils/SkThreadPool.h
index 6cb8b528d316624bb388b22b09eea132178fc251..0aa7c08ad53b6e48d8f82ce61b95dca36fdc679e 100644
--- a/include/utils/SkThreadPool.h
+++ b/include/utils/SkThreadPool.h
@@ -45,10 +45,17 @@ public:
         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.
 };