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/flags/SkCommonFlags.cpp b/tools/flags/SkCommonFlags.cpp
index a2da131..2d5a855 100644
--- a/tools/flags/SkCommonFlags.cpp
+++ b/tools/flags/SkCommonFlags.cpp
@@ -48,3 +48,4 @@
 DEFINE_string(properties, "",
               "Space-separated key/value pairs to add to JSON identifying this run.");
 
+DEFINE_bool2(pre_log, p, false, "Log before running each test. May be incomprehensible when threading");