Fix format of nanobench result

Column of samples is too wide. This makes config is not align with the
'config' title. Pad 'samples' tilte with some whitespaces to fix this
issue.

BUG=skia:
R=mtklein@google.com

Author: qiankun.miao@intel.com

Review URL: https://codereview.chromium.org/549153005
diff --git a/bench/nanobench.cpp b/bench/nanobench.cpp
index a8274d8..20afdeb 100644
--- a/bench/nanobench.cpp
+++ b/bench/nanobench.cpp
@@ -582,7 +582,8 @@
     } else if (FLAGS_quiet) {
         SkDebugf("median\tbench\tconfig\n");
     } else {
-        SkDebugf("maxrss\tloops\tmin\tmedian\tmean\tmax\tstddev\tsamples\tconfig\tbench\n");
+        SkDebugf("maxrss\tloops\tmin\tmedian\tmean\tmax\tstddev\t%-*s\tconfig\tbench\n",
+                 FLAGS_samples, "samples");
     }
 
     SkTDArray<Config> configs;