[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..72b843dc9ffc765938f73116d1f5a49c1f27f4cd 100644
--- a/components/history/core/browser/history_model_worker.cc
+++ b/components/history/core/browser/history_model_worker.cc
@@ -124,6 +124,13 @@ syncer::ModelSafeGroup HistoryModelWorker::GetModelSafeGroup() {
   return syncer::GROUP_HISTORY;
 }
 
+bool HistoryModelWorker::IsOnModelThread() {
+  // Ideally HistoryService would expose a way to check whether this is the
+  // history DB thread. Since it doesn't, just return true to bypass a CHECK in

[sky, 2016/11/08 23:06:50]

You actually could add such a check, but there is work going on to move
HistoryService to a TaskScheduler, which would mean said check wouldn't work.
That bug is here: https://codereview.chromium.org/2486603003/ . I'm adding you
to that bug in case there are issues with it and sync.

+  // the sync code.
+  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