sync: Refactor per-datatype throttling

sync/engine/syncer.cc

Index: sync/engine/syncer.cc
diff --git a/sync/engine/syncer.cc b/sync/engine/syncer.cc
index 1a299db5e27c03f26753aa3e26ab4467db99ed51..c17c0f8e6084f3f29b71081d0561af43f3751618 100644
--- a/sync/engine/syncer.cc
+++ b/sync/engine/syncer.cc
@@ -24,6 +24,7 @@
 #include "sync/engine/store_timestamps_command.h"
 #include "sync/engine/syncer_types.h"
 #include "sync/engine/syncproto.h"
+#include "sync/engine/throttled_data_type_tracker.h"
 #include "sync/engine/verify_updates_command.h"
 #include "sync/syncable/syncable-inl.h"
 #include "sync/syncable/syncable.h"
@@ -107,7 +108,8 @@ void Syncer::SyncShare(sessions::SyncSession* session,
 
     switch (current_step) {
       case SYNCER_BEGIN:
-        session->context()->PruneUnthrottledTypes(base::TimeTicks::Now());
+        session->context()->throttled_data_type_tracker()->
+            PruneUnthrottledTypes(base::TimeTicks::Now());
         session->SendEventNotification(SyncEngineEvent::SYNC_CYCLE_BEGIN);
 
         next_step = CLEANUP_DISABLED_TYPES;