[test] Print how test runner is called on swarming.

tools/run-tests.py

Index: tools/run-tests.py
diff --git a/tools/run-tests.py b/tools/run-tests.py
index 3d143ac6adb5cd11869299bca6d6bb2cd4830e94..fb32c28799677d59684053b3556422a061a36c57 100755
--- a/tools/run-tests.py
+++ b/tools/run-tests.py
@@ -597,6 +597,11 @@ def Main():
     return 1
   SetupEnvironment(options)
 
+  if options.swarming:
+    # Swarming doesn't print how isolated commands are called. Lets make this
+    # less cryptic by printing it ourselves.
+    print ' '.join(sys.argv)
+
   exit_code = 0
   if not options.no_presubmit:
     print ">>> running presubmit tests"