Add tools/nanobench_flags.py.

This should look suspiciously similar to tools/dm_flags.py.  In fact, I
tweaked tools/dm_flags.py a bit to make it even more suspiciously similar.
I'll leave actually deduping this to future me.

I noticed we have an opportunity to make our Valgrind run of nanobench faster,
by not only making it not auto-calibrate (--loops 1) but also take only one
measurement (--samples 1).  Should be 5-10x faster than the default.

BUG=skia:

Review URL: https://codereview.chromium.org/957503002
diff --git a/tools/dm_flags.py b/tools/dm_flags.py
index 277bf8a..985a62a 100755
--- a/tools/dm_flags.py
+++ b/tools/dm_flags.py
@@ -19,7 +19,7 @@
 
 
 cov_start = lineno()+1   # We care about coverage starting just past this def.
-def get_dm_args(bot):
+def get_args(bot):
   args = []
 
   configs = ['565', '8888', 'gpu', 'nvprmsaa4']
@@ -94,7 +94,7 @@
   cov = coverage.coverage()
   cov.start()
   for case in cases:
-    args[case] = get_dm_args(case)
+    args[case] = get_args(case)
   cov.stop()
 
   this_file = os.path.basename(__file__)
@@ -119,4 +119,4 @@
     sys.exit(1)
 
   with open(sys.argv[1], 'w') as out:
-    json.dump(get_dm_args(sys.argv[2]), out)
+    json.dump(get_args(sys.argv[2]), out)