Keep valgrind happen by purging caches.

Change-Id: Ic387edb9b5d71110c10c8a40a362ed2853d140f5
Reviewed-on: https://skia-review.googlesource.com/7638
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
diff --git a/bench/nanobench.cpp b/bench/nanobench.cpp
index 4a058be..6aea7b5 100644
--- a/bench/nanobench.cpp
+++ b/bench/nanobench.cpp
@@ -1364,6 +1364,8 @@
         }
     }
 
+    SkGraphics::PurgeAllCaches();
+
     log->bench("memory_usage", 0,0);
     log->config("meta");
     log->metric("max_rss_mb", sk_tools::getMaxResidentSetSizeMB());