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>

bench/nanobench.cpp

@@ -1364,6 +1364,8 @@ int nanobench_main() {
         }
     }
 
+    SkGraphics::PurgeAllCaches();
+
     log->bench("memory_usage", 0,0);
     log->config("meta");
     log->metric("max_rss_mb", sk_tools::getMaxResidentSetSizeMB());

dm/DM.cpp

@@ -1382,6 +1382,7 @@ int dm_main() {
 #endif  // SK_PDF_IMAGE_STATS
 
     print_status();
+    SkGraphics::PurgeAllCaches();
     info("Finished!\n");
     return 0;
 }