Add a new stage CanaryCompletionStage

Canary master sends out emails to summarize failures in this stage.

BUG=chromium:398262
TEST=`cbuildbot/run_tests`
TEST=`cbuildbot --remote --debug --buildbot master-realse`

Change-Id: Ie5bbd64cfb9d582fbdd36e5ae4e566dc25af91a4
Reviewed-on: https://chromium-review.googlesource.com/211499
Reviewed-by: Yu-Ju Hong <yjhong@chromium.org>
Commit-Queue: Yu-Ju Hong <yjhong@chromium.org>
Tested-by: Yu-Ju Hong <yjhong@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index fe6d4a7..b9f14a9 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -749,7 +749,7 @@
       sync_stage = self._GetStageInstance(
           sync_stages.ManifestVersionedSyncStage)
       self.completion_stage_class = (
-          completion_stages.MasterSlaveSyncCompletionStage)
+          completion_stages.CanaryCompletionStage)
     else:
       sync_stage = self._GetStageInstance(
           sync_stages.ManifestVersionedSyncStage)