sync: make scheduling logic and job ownership more obvious.

sync/engine/syncer.h

Index: sync/engine/syncer.h
diff --git a/sync/engine/syncer.h b/sync/engine/syncer.h
index 8fadde9ecb74a09878bb518e5b7a6ddcd0b26c8e..88790a15b9e5f30e213f3d0ee79873b6bcbe818d 100644
--- a/sync/engine/syncer.h
+++ b/sync/engine/syncer.h
@@ -60,7 +60,9 @@ class Syncer {
   void RequestEarlyExit();
 
   // Runs a sync cycle from |first_step| to |last_step|.
-  virtual void SyncShare(sessions::SyncSession* session,
+  // Returns true if the cycle completed with |last_step|, and false
+  // if it terminated early due to error / exit requested.
+  virtual bool SyncShare(sessions::SyncSession* session,
                          SyncerStep first_step,
                          SyncerStep last_step);