Fix py3 presubmit error. Use unicode strings explicitly.

Bug:1225052
Change-Id: I7c536819133c75130baca2f8d295360d7d1ca69c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2995072
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Kyle Ju <kyleju@chromium.org>

presubmit_support.py

@@ -331,7 +331,7 @@ class _PresubmitResult(object):
     if self._long_text:
       sys.stdout.write('\n***************\n')
       # Write separately in case it's unicode.
-      sys.stdout.write(self._long_text)
+      sys.stdout.write(str(self._long_text))
       sys.stdout.write('\n***************\n')
 
   def json_format(self):