SkThreadPool: tweak two little things that have been annoying me

include/utils/SkThreadPool.h

Index: include/utils/SkThreadPool.h
diff --git a/include/utils/SkThreadPool.h b/include/utils/SkThreadPool.h
index 98657034e58bec72e90080df5800ccfed6a32612..6cb8b528d316624bb388b22b09eea132178fc251 100644
--- a/include/utils/SkThreadPool.h
+++ b/include/utils/SkThreadPool.h
@@ -9,10 +9,10 @@
 #define SkThreadPool_DEFINED
 
 #include "SkCondVar.h"
+#include "SkRunnable.h"
 #include "SkTDArray.h"
 #include "SkTInternalLList.h"
 
-class SkRunnable;
 class SkThread;
 
 class SkThreadPool {
@@ -31,6 +31,11 @@ public:
      */
     void add(SkRunnable*);
 
+    /**
+     * Block until all added SkRunnables have completed.  Once called, calling add() is undefined.
+     */
+    void wait();
+
  private:
     struct LinkedRunnable {
         // Unowned pointer.