Add missing method stub for _GerritCodereviewImpl.

This should make presubmit happy for oncoming git cl desc for Gerrit:
https://codereview.chromium.org/1917473002/

BUG=603207
TBR=hinoka@chromium.org,scottmg@chromium.org

Review URL: https://codereview.chromium.org/1917933003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@300186 0039d316-1c4b-4281-b951-d872f2087c98

git_cl.py

@@ -2175,6 +2175,13 @@ class _GerritChangelistImpl(_ChangelistCodereviewBase):
   def CloseIssue(self):
     gerrit_util.AbandonChange(self._GetGerritHost(), self.GetIssue(), msg='')
 
+  def GetApprovingReviewers(self):
+    """Returns a list of reviewers approving the change.
+
+    Note: not necessarily committers.
+    """
+    raise NotImplementedError()
+
   def SubmitIssue(self, wait_for_merge=True):
     gerrit_util.SubmitChange(self._GetGerritHost(), self.GetIssue(),
                              wait_for_merge=wait_for_merge)