Add missing method stub for _GerritCodereviewImpl.
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: fd409f4bdeea2bb50a5d34bb4d4bfc2046a5a3dd@300186 0039d316-1c4b-4281-b951-d872f2087c98

primiano/depot_tools,CoherentLabs/depot_tools,primiano/depot_tools,CoherentLabs/depot_tools,primiano/depot_tools