[Sync] Add ClearServerData support to Sync Scheduler

sync/engine/syncer.h

Index: sync/engine/syncer.h
diff --git a/sync/engine/syncer.h b/sync/engine/syncer.h
index e473bae86381349629d1028ee75c92fce8e898a9..7b77d07c1933c3d690f20880431a29cf779a694b 100644
--- a/sync/engine/syncer.h
+++ b/sync/engine/syncer.h
@@ -78,6 +78,11 @@ class SYNC_EXPORT_PRIVATE Syncer {
   virtual bool PollSyncShare(ModelTypeSet request_types,
                              sessions::SyncSession* session);
 
+  // Posts a ClearServerData command.
+  // Returns: false if an error occurred and retries should backoff, true
+  // otherwise.
+  virtual bool PostClearServerData(sessions::SyncSession* session);
+
  private:
   friend class SyncerTest;
   FRIEND_TEST_ALL_PREFIXES(SyncerTest, NameClashWithResolver);