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 7efd858c5cd5a8a0e761c399a839fa6964526e2b..bc3bbb45234689ca69e077c387ce75d59d258059 100644
--- a/bench/nanobench.cpp
+++ b/bench/nanobench.cpp
@@ -1119,6 +1119,12 @@ 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);