Implement a benchmark for GrResourceCache

bench/MutexBench.cpp

Index: bench/MutexBench.cpp
diff --git a/bench/MutexBench.cpp b/bench/MutexBench.cpp
index 1d037ec1cd27da247e63c012c641361315918dc5..bec0c8058c92b0f39ff3d094039531847c205ea1 100644
--- a/bench/MutexBench.cpp
+++ b/bench/MutexBench.cpp
@@ -9,9 +9,10 @@
 
 class MutexBench : public SkBenchmark {
 public:
-    MutexBench()  {
-        fIsRendering = false;
+    virtual bool isSuitableFor(Backend backend) SK_OVERRIDE {
+        return backend == kNonRendering_Backend;
     }
+
 protected:
     virtual const char* onGetName() {
         return "mutex";