nanobench: support GMs-as-benches

BUG=skia:
R=jcgregorio@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/438683002
diff --git a/bench/GMBench.cpp b/bench/GMBench.cpp
index 41a8e8c..dbd1d7b 100644
--- a/bench/GMBench.cpp
+++ b/bench/GMBench.cpp
@@ -8,7 +8,7 @@
 #include "GMBench.h"
 
 GMBench::GMBench(skiagm::GM* gm) : fGM(gm) {
-    fName.printf("GM:%s", gm->getName());
+    fName.printf("GM_%s", gm->getName());
 }
 
 GMBench::~GMBench() { delete fGM; }