Revert 112815 - [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
===================================================================
--- chrome/browser/sync/engine/model_changing_syncer_command.cc	(revision 112854)
+++ chrome/browser/sync/engine/model_changing_syncer_command.cc	(working copy)
@@ -6,6 +6,7 @@
 
 #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"
@@ -18,13 +19,27 @@
     return;
   }
 
-  const std::set<ModelSafeGroup>& groups_to_change =
-      GetGroupsToChange(*work_session_);
+  // 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);
+
   for (size_t i = 0; i < session->workers().size(); ++i) {
-    ModelSafeWorker* worker = work_session_->workers()[i];
+    ModelSafeWorker* worker = session->workers()[i];
     ModelSafeGroup group = worker->GetModelSafeGroup();
     // Skip workers whose group isn't active.
-    if (groups_to_change.count(group) == 0u) {
+    if (active_groups.find(group) == active_groups.end()) {
       DVLOG(2) << "Skipping worker for group "
                << ModelSafeGroupToString(group);
       continue;