[Sync] Make syncer commands avoid posting tasks on threads with no work to do

chrome/browser/sync/engine/model_changing_syncer_command.cc

Index: chrome/browser/sync/engine/model_changing_syncer_command.cc
diff --git a/chrome/browser/sync/engine/model_changing_syncer_command.cc b/chrome/browser/sync/engine/model_changing_syncer_command.cc
index a8df65ade9e3eb0795ef41c8bc0de775e5e136d4..a4aa26e15bda636f46c412fb76e948a5128411ac 100644
--- a/chrome/browser/sync/engine/model_changing_syncer_command.cc
+++ b/chrome/browser/sync/engine/model_changing_syncer_command.cc
@@ -6,7 +6,6 @@
 
 #include "base/basictypes.h"
 #include "base/callback_old.h"
-#include "chrome/browser/sync/engine/model_safe_worker.h"
 #include "chrome/browser/sync/sessions/status_controller.h"
 #include "chrome/browser/sync/sessions/sync_session.h"
 #include "chrome/browser/sync/util/unrecoverable_error_info.h"
@@ -19,27 +18,15 @@ void ModelChangingSyncerCommand::ExecuteImpl(sessions::SyncSession* session) {
     return;
   }
 
-  // Project the list of active types (i.e., types in the routing
-  // info) to a list of groups.
-  //
-  // TODO(akalin): Make this overrideable by subclasses (who might be
-  // working on a subset of |active_groups|).  (See
-  // http://crbug.com/97832.)
-  std::set<ModelSafeGroup> active_groups;
-  const ModelSafeRoutingInfo& routing_info = session->routing_info();
-  for (ModelSafeRoutingInfo::const_iterator it = routing_info.begin();
-       it != routing_info.end(); ++it) {
-    active_groups.insert(it->second);
-  }
-  // Always work on GROUP_PASSIVE, since that's the group that
-  // top-level folders map to.
-  active_groups.insert(GROUP_PASSIVE);
-
+  const std::set<ModelSafeGroup>& groups_to_change =
+      HasCustomGroupsToChange() ?
+      GetGroupsToChange(*work_session_) :
+      session->GetEnabledGroups();
   for (size_t i = 0; i < session->workers().size(); ++i) {
-    ModelSafeWorker* worker = session->workers()[i];
+    ModelSafeWorker* worker = work_session_->workers()[i];
     ModelSafeGroup group = worker->GetModelSafeGroup();
     // Skip workers whose group isn't active.
-    if (active_groups.find(group) == active_groups.end()) {
+    if (groups_to_change.count(group) == 0u) {
       DVLOG(2) << "Skipping worker for group "
                << ModelSafeGroupToString(group);
       continue;