presubmit_support: Add support for setting the change description.

Needed by Skia's PostUploadHooks, so that presubmit_support
doesn't depend on git-cl classes.

Bug: 1042324
Change-Id: I02475e346437b26fa8d0d2021ea62754077a2b5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2007262Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

presubmit_support.py

@@ -480,6 +480,9 @@ class GerritAccessor(object):
       reviewers = changeinfo.get('reviewers', {}).get('REVIEWER', [])
     return [r.get('email') for r in reviewers]
 
+  def UpdateDescription(self, description, issue):
+    gerrit_util.SetCommitMessage(self.host, issue, description, notify='NONE')
+
 
 class OutputApi(object):
   """An instance of OutputApi gets passed to presubmit scripts so that they