move mutex construction out of the benchmarks.

bench/MutexBench.cpp

Index: bench/MutexBench.cpp
diff --git a/bench/MutexBench.cpp b/bench/MutexBench.cpp
index 6dd4e5346f7dbf11a826431dc41ec3187b2b5448..ba92a43033871c045dd39a3c9e33b7adc6d3cf1b 100644
--- a/bench/MutexBench.cpp
+++ b/bench/MutexBench.cpp
@@ -24,16 +24,16 @@ protected:
     }
 
     void onDraw(const int loops, SkCanvas*) override {
-        Mutex mu;
         for (int i = 0; i < loops; i++) {
-            mu.acquire();
-            mu.release();
+            fMu.acquire();
+            fMu.release();
         }
     }
 
 private:
     typedef Benchmark INHERITED;
     SkString fBenchName;
+    Mutex fMu;
 };
 
 class SharedBench : public Benchmark {
@@ -48,15 +48,15 @@ protected:
     }
 
     void onDraw(const int loops, SkCanvas*) override {
-        SkSharedMutex mu;
         for (int i = 0; i < loops; i++) {
-            mu.acquireShared();
-            mu.releaseShared();
+            fMu.acquireShared();
+            fMu.releaseShared();
         }
     }
 
 private:
     typedef Benchmark INHERITED;
+    SkSharedMutex fMu;
 };
 
 ///////////////////////////////////////////////////////////////////////////////