Enable SkTaskGroup threading in monobench.

tools/monobench.cpp

 /*
  * Copyright 2016 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 "SkGraphics.h"
+#include "SkTaskGroup.h"
 #include <algorithm>
 #include <chrono>
 #include <regex>
 #include <stdio.h>
 #include <string>
 #include <vector>
 
 int main(int argc, char** argv) {
     SkGraphics::Init();
+    SkTaskGroup::Enabler enabled;
 
     using clock = std::chrono::high_resolution_clock;
     using ns = std::chrono::duration<double, std::nano>;
 
     std::regex pattern;
     if (argc > 1) {
         pattern = argv[1];
     }
 
     struct Bench {
@@ skipping 76 matching lines @@
                     }
                 }
                 fflush(stdout);
                 break;
             }
         }
     }
 
     return 0;
 }