sync: Add interfaces for per-type sync

sync/sync_core.gypi

Index: sync/sync_core.gypi
diff --git a/sync/sync_core.gypi b/sync/sync_core.gypi
index a3dd9aa7016be72c2d0f3497247bde87537f8a91..5aa74f6e6742284a2aa29d9fcab2d91135512c69 100644
--- a/sync/sync_core.gypi
+++ b/sync/sync_core.gypi
@@ -35,6 +35,10 @@
     'engine/backoff_delay_provider.cc',
     'engine/backoff_delay_provider.h',
     'engine/commit.cc',
+    'engine/commit_contribution.cc',
+    'engine/commit_contribution.h',
+    'engine/commit_contributor.cc',
+    'engine/commit_contributor.h',
     'engine/commit.h',
     'engine/commit_processor.cc',
     'engine/commit_processor.h',
@@ -44,6 +48,12 @@
     'engine/conflict_resolver.h',
     'engine/conflict_util.cc',
     'engine/conflict_util.h',
+    'engine/directory_commit_contribution.cc',
+    'engine/directory_commit_contribution.h',
+    'engine/directory_commit_contributor.cc',
+    'engine/directory_commit_contributor.h',
+    'engine/directory_update_handler.cc',
+    'engine/directory_update_handler.h',
     'engine/download.cc',
     'engine/download.h',
     'engine/get_commit_ids.cc',
@@ -62,18 +72,8 @@
     'engine/process_updates_util.h',
     'engine/sync_cycle_event.cc',
     'engine/sync_cycle_event.h',
-    'engine/sync_directory_commit_contribution.cc',
-    'engine/sync_directory_commit_contribution.h',
-    'engine/sync_directory_commit_contributor.cc',
-    'engine/sync_directory_commit_contributor.h',
-    'engine/sync_directory_update_handler.cc',
-    'engine/sync_directory_update_handler.h',
     'engine/sync_engine_event_listener.cc',
     'engine/sync_engine_event_listener.h',
-    'engine/sync_scheduler.cc',
-    'engine/sync_scheduler.h',
-    'engine/sync_scheduler_impl.cc',
-    'engine/sync_scheduler_impl.h',
     'engine/syncer.cc',
     'engine/syncer.h',
     'engine/syncer_proto_util.cc',
@@ -81,12 +81,18 @@
     'engine/syncer_types.h',
     'engine/syncer_util.cc',
     'engine/syncer_util.h',
+    'engine/sync_scheduler.cc',
+    'engine/sync_scheduler.h',
+    'engine/sync_scheduler_impl.cc',
+    'engine/sync_scheduler_impl.h',
     'engine/traffic_logger.cc',
     'engine/traffic_logger.h',
     'engine/traffic_recorder.cc',
     'engine/traffic_recorder.h',
     'engine/update_applicator.cc',
     'engine/update_applicator.h',
+    'engine/update_handler.cc',
+    'engine/update_handler.h',
     'js/js_arg_list.cc',
     'js/js_arg_list.h',
     'js/js_backend.h',