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

chrome/browser/sync/engine/process_commit_response_command.cc

Index: chrome/browser/sync/engine/process_commit_response_command.cc
diff --git a/chrome/browser/sync/engine/process_commit_response_command.cc b/chrome/browser/sync/engine/process_commit_response_command.cc
index 7a7113f89c4b206e2ab3f093cbc3171ed5cd1b0c..23b89772a3a97e35a05aec50c2492f89b2b18798 100644
--- a/chrome/browser/sync/engine/process_commit_response_command.cc
+++ b/chrome/browser/sync/engine/process_commit_response_command.cc
@@ -4,6 +4,7 @@
 
 #include "chrome/browser/sync/engine/process_commit_response_command.h"
 
+#include <cstddef>
 #include <set>
 #include <string>
 #include <vector>
@@ -59,6 +60,27 @@ void ResetErrorCounters(StatusController* status) {
 ProcessCommitResponseCommand::ProcessCommitResponseCommand() {}
 ProcessCommitResponseCommand::~ProcessCommitResponseCommand() {}
 
+std::set<ModelSafeGroup> ProcessCommitResponseCommand::GetGroupsToChange(
+    const sessions::SyncSession& session) const {
+  std::set<ModelSafeGroup> groups_with_commits;
+  syncable::ScopedDirLookup dir(session.context()->directory_manager(),
+                                session.context()->account_name());
+  if (!dir.good()) {
+    LOG(ERROR) << "Scoped dir lookup failed!";
+    return groups_with_commits;
+  }
+
+  syncable::ReadTransaction trans(FROM_HERE, dir);
+  const StatusController& status = session.status_controller();
+  for (size_t i = 0; i < status.commit_ids().size(); ++i) {
+    groups_with_commits.insert(
+        GetGroupForModelType(status.GetUnrestrictedCommitModelTypeAt(i),
+                             session.routing_info()));
+  }
+
+  return groups_with_commits;
+}
+
 bool ProcessCommitResponseCommand::ModelNeutralExecuteImpl(
     sessions::SyncSession* session) {
   ScopedDirLookup dir(session->context()->directory_manager(),