Add pre_log option to nanobench; make this option default true in nanobench and dm.

bench/nanobench.cpp

Index: bench/nanobench.cpp
diff --git a/bench/nanobench.cpp b/bench/nanobench.cpp
index 4fc4044449b5e65a9a661133bef72520ff28903d..cfedc52379d21f5b39a255696cdcd3a5b71aa369 100644
--- a/bench/nanobench.cpp
+++ b/bench/nanobench.cpp
@@ -1124,6 +1124,12 @@ int nanobench_main() {
             SkCanvas* canvas = target->getCanvas();
             const char* config = target->config.name;
 
+            if (FLAGS_pre_log) {
+                SkDebugf("Running %s\t%s\n"
+                         , bench->getUniqueName()
+                         , config);
+            }
+
             target->setup();
             bench->perCanvasPreDraw(canvas);