Fix up -Winconsistent-missing-override

bench/MutexBench.cpp

Index: bench/MutexBench.cpp
diff --git a/bench/MutexBench.cpp b/bench/MutexBench.cpp
index 8d7eb985460db268dd2c716f1efa06adf71125b8..2565d3860e8f5e77bc4486c490acc1a0735b5faa 100644
--- a/bench/MutexBench.cpp
+++ b/bench/MutexBench.cpp
@@ -14,11 +14,11 @@ public:
     }
 
 protected:
-    virtual const char* onGetName() {
+    const char* onGetName() override {
         return "mutex";
     }
 
-    virtual void onDraw(const int loops, SkCanvas*) {
+    void onDraw(const int loops, SkCanvas*) override {
         SkMutex mu;
         for (int i = 0; i < loops; i++) {
             mu.acquire();