sync: Implement per-type update processing

sync/sessions/sync_session_context.h

Index: sync/sessions/sync_session_context.h
diff --git a/sync/sessions/sync_session_context.h b/sync/sessions/sync_session_context.h
index 3133c32a2ef3e4b85e76aed0a3480aba4d09b28c..d195943686e9068339ce4cecb512fcd157e890d6 100644
--- a/sync/sessions/sync_session_context.h
+++ b/sync/sessions/sync_session_context.h
@@ -25,6 +25,7 @@
 #include "base/stl_util.h"
 #include "sync/base/sync_export.h"
 #include "sync/engine/sync_directory_commit_contributor.h"
+#include "sync/engine/sync_directory_update_handler.h"
 #include "sync/engine/sync_engine_event.h"
 #include "sync/engine/syncer_types.h"
 #include "sync/engine/traffic_recorder.h"
@@ -75,6 +76,10 @@ class SYNC_EXPORT_PRIVATE SyncSessionContext {
 
   void set_routing_info(const ModelSafeRoutingInfo& routing_info);
 
+  UpdateHandlerMap* update_handler_map() {
+    return &update_handler_map_;
+  }
+
   CommitContributorMap* commit_contributor_map() {
     return &commit_contributor_map_;
   }
@@ -158,6 +163,14 @@ class SYNC_EXPORT_PRIVATE SyncSessionContext {
   // Must be updated manually when SBR's state is modified.
   ModelSafeRoutingInfo routing_info_;
 
+  // A map of 'update handlers', one for each enabled type.
+  // This must be kept in sync with the routing info.  Our temporary solution to
+  // that problem is to initialize this map in set_routing_info().
+  UpdateHandlerMap update_handler_map_;
+
+  // Deleter for the |update_handler_map_|.
+  STLValueDeleter<UpdateHandlerMap> update_handler_deleter_;
+
   // A map of 'commit contributors', one for each enabled type.
   // This must be kept in sync with the routing info.  Our temporary solution to
   // that problem is to initialize this map in set_routing_info().