Implement a benchmark for GrResourceCache

bench/SkBenchmark.cpp

Index: bench/SkBenchmark.cpp
diff --git a/bench/SkBenchmark.cpp b/bench/SkBenchmark.cpp
index 432d5be6432e87771702a05a3c7aefb9d94e0f60..4327665d06dd60c28cd00fcb7c71995dd5eaf60f 100644
--- a/bench/SkBenchmark.cpp
+++ b/bench/SkBenchmark.cpp
@@ -21,7 +21,9 @@ SkBenchmark::SkBenchmark() {
     fForceAA = true;
     fForceFilter = false;
     fDither = SkTriState::kDefault;
-    fIsRendering = true;
+#if SK_SUPPORT_GPU
+    fContext = NULL;
+#endif
     fOrMask = fClearMask = 0;
     fLoops = 1;
 }