sync: Add interfaces for per-type sync

sync/engine/commit_processor.cc

Index: sync/engine/commit_processor.cc
diff --git a/sync/engine/commit_processor.cc b/sync/engine/commit_processor.cc
index c54217b0954534ebb1add5f86ad4afee1f489ed4..16afaa70d51a6012788bf26fb6e435f6f989e104 100644
--- a/sync/engine/commit_processor.cc
+++ b/sync/engine/commit_processor.cc
@@ -6,8 +6,8 @@
 
 #include <map>
 
-#include "sync/engine/sync_directory_commit_contribution.h"
-#include "sync/engine/sync_directory_commit_contributor.h"
+#include "sync/engine/commit_contribution.h"
+#include "sync/engine/commit_contributor.h"
 #include "sync/protocol/sync.pb.h"
 
 namespace syncer {
@@ -35,7 +35,7 @@ void CommitProcessor::GatherCommitContributions(
       continue;
     }
     size_t spaces_remaining = max_entries - num_entries;
-    SyncDirectoryCommitContribution* contribution =
+    CommitContribution* contribution =
         cm_it->second->GetContribution(spaces_remaining);
     if (contribution) {
       num_entries += contribution->GetNumEntries();