Add shared mutex benchmark.

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 "SkSharedMutex.h"
 #include "SkSpinlock.h"
@@ skipping 18 matching lines @@
             mu.acquire();
             mu.release();
         }
     }
 
 private:
     typedef Benchmark INHERITED;
     SkString fBenchName;
 };
 
+class SharedBench : public Benchmark {
+public:
+    bool isSuitableFor(Backend backend) override {
+        return backend == kNonRendering_Backend;
+    }
+
+protected:
+    const char* onGetName() override {
+        return "SkSharedMutexSharedUncontendedBenchmark";
+    }
+
+    void onDraw(const int loops, SkCanvas*) override {
+        SkSharedMutex mu;
+        for (int i = 0; i < loops; i++) {
+            mu.acquireShared();
+            mu.releaseShared();
+        }
+    }
+
+private:
+    typedef Benchmark INHERITED;
+};
+
 ///////////////////////////////////////////////////////////////////////////////
 
 DEF_BENCH( return new MutexBench<SkSharedMutex>(SkString("SkSharedMutex")); )
 DEF_BENCH( return new MutexBench<SkMutex>(SkString("SkMutex")); )
 DEF_BENCH( return new MutexBench<SkSpinlock>(SkString("SkSpinlock")); )
+DEF_BENCH( return new SharedBench; )
+