Revert of SkThreadPool ~~> SkTaskGroup

src/utils/SkTaskGroup.h

-/*
- * Copyright 2014 Google Inc.
- *
- * Use of this source code is governed by a BSD-style license that can be
- * found in the LICENSE file.
- */
-
-#ifndef SkTaskGroup_DEFINED
-#define SkTaskGroup_DEFINED
-
-#include "SkTypes.h"
-#include "SkRunnable.h"
-
-class SkTaskGroup : SkNoncopyable {
-public:
-    // Create one of these in main() to enable SkTaskGroups globally.
-    struct Enabler : SkNoncopyable {
-        explicit Enabler(int threads = 0);  // Default is system-reported core count.
-        ~Enabler();
-    };
-
-    SkTaskGroup();
-    ~SkTaskGroup() { this->wait(); }
-
-    // Add a task to this SkTaskGroup.  It will likely run() on another thread.
-    void add(SkRunnable*);
-
-    // Block until all Tasks previously add()ed to this SkTaskGroup have run().
-    // You may safely reuse this SkTaskGroup after wait() returns.
-    void wait();
-
-private:
-    /*atomic*/ int32_t fPending;
-};
-
-#endif//SkTaskGroup_DEFINED