Add pre_log option to nanobench; make this option default true in nanobench_flags.py and dm_flags.py.

BUG=skia:4826
BUG=skia:4827
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1612483002

Review URL: https://codereview.chromium.org/1612483002
diff --git a/tools/dm_flags.py b/tools/dm_flags.py
index 8d74450..be348bc 100755
--- a/tools/dm_flags.py
+++ b/tools/dm_flags.py
@@ -27,7 +27,7 @@
 
 cov_start = lineno()+1   # We care about coverage starting just past this def.
 def get_args(bot):
-  args = []
+  args = ['--pre_log']
 
   configs = ['565', '8888', 'gpu']