[Sync] Move thread checking into the ModelSafeWorker interface.

components/history/core/browser/history_model_worker.cc

Index: components/history/core/browser/history_model_worker.cc
diff --git a/components/history/core/browser/history_model_worker.cc b/components/history/core/browser/history_model_worker.cc
index 54828f28340223698cc011a0904b690cd52d395c..20ddbe20b890f1d1ccc018d2979cb67339d62f25 100644
--- a/components/history/core/browser/history_model_worker.cc
+++ b/components/history/core/browser/history_model_worker.cc
@@ -124,6 +124,11 @@ syncer::ModelSafeGroup HistoryModelWorker::GetModelSafeGroup() {
   return syncer::GROUP_HISTORY;
 }
 
+bool HistoryModelWorker::IsOnModelThread() {
+  // Can't do better without modifying HistoryService.

[sky, 2016/11/08 18:17:00]

As someone not familiar with this function the comment doesn't make sense. I
recommend removing the comment, or better documenting what you mean. Ideally the
comment would also way what true means.

[maxbogue, 2016/11/08 18:43:23]

Improved; let me know if anything is still unclear!

+  return true;
+}
+
 HistoryModelWorker::~HistoryModelWorker() {
   // The base::CancelableTaskTracker class is not thread-safe and must only be
   // used from a single thread but the current object may not be destroyed from