[Sync] Add support for enabling session sync remotely.

chrome/browser/sync/glue/sync_backend_host.cc

Index: chrome/browser/sync/glue/sync_backend_host.cc
diff --git a/chrome/browser/sync/glue/sync_backend_host.cc b/chrome/browser/sync/glue/sync_backend_host.cc
index efa717b1d97cdac7119bb3f4691b24a8436caecf..99308618b06f8511a50ee8e624ee50485e8de684 100644
--- a/chrome/browser/sync/glue/sync_backend_host.cc
+++ b/chrome/browser/sync/glue/sync_backend_host.cc
@@ -867,6 +867,14 @@ void SyncBackendHost::Core::HandleSyncCycleCompletedOnFrontendLoop(
   if (!to_migrate.empty())
     host_->frontend_->OnMigrationNeededForTypes(to_migrate);
 
+  // Process any changes to the datatypes we're syncing.
+  // TODO(sync): add support for removing types.
+  syncable::ModelTypeSet to_add;
+  if (sync_manager()->ReceivedExperimentalTypes(&to_add) &&
+      host_->initialized()) {
+    host_->frontend_->OnDataTypesChanged(to_add);
+  }
+
   // If we are waiting for a configuration change, check here to see
   // if this sync cycle has initialized all of the types we've been
   // waiting for.