Revert of Remove SkThread.h, include SkMutex.h or SkAtomics.h as appropriate.

bench/MutexBench.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 #include "Benchmark.h"
-#include "SkMutex.h"
+#include "SkThread.h"
 
 class MutexBench : public Benchmark {
 public:
     bool isSuitableFor(Backend backend) override {
         return backend == kNonRendering_Backend;
     }
 
 protected:
     virtual const char* onGetName() {
         return "mutex";
     }
 
     virtual void onDraw(const int loops, SkCanvas*) {
         SkMutex mu;
         for (int i = 0; i < loops; i++) {
             mu.acquire();
             mu.release();
         }
     }
 
 private:
     typedef Benchmark INHERITED;
 };
 
 ///////////////////////////////////////////////////////////////////////////////
 
 DEF_BENCH( return new MutexBench(); )