[Sync] Move thread checking into the ModelSafeWorker interface.

components/password_manager/sync/browser/password_model_worker.cc

Index: components/password_manager/sync/browser/password_model_worker.cc
diff --git a/components/password_manager/sync/browser/password_model_worker.cc b/components/password_manager/sync/browser/password_model_worker.cc
index 91d17c883f50439c547f33cd8ad18acbe796b24f..7a22023f3b618ab73bdb995c65772cdb1c11db39 100644
--- a/components/password_manager/sync/browser/password_model_worker.cc
+++ b/components/password_manager/sync/browser/password_model_worker.cc
@@ -62,6 +62,11 @@ syncer::ModelSafeGroup PasswordModelWorker::GetModelSafeGroup() {
   return syncer::GROUP_PASSWORD;
 }
 
+bool PasswordModelWorker::IsOnModelThread() {
+  // Can't do better without modifying PasswordStore.

[vabr (Chromium), 2016/11/08 08:32:45]

Please explain the issue better. Ideally, the issue would be described in a bug
in more detail (what needs to be done in PasswordStore, how it relates to the
worker, etc.), and the comment here could just say: TODO(crbug.com/XXX) Can't do
better without modifying PasswordStore.

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

I've improved the comment to hopefully be clearer about what's going on. I'm not
filing a bug because nobody actually intends to fix this; it's sat untouched in
sync code for years and we're beginning to move types to the new USS style,
which doesn't use this interface.

[vabr (Chromium), 2016/11/08 19:24:59]

Acknowledged. If there was a need to implement this in PasswordStore, do let me
know, it should not be a problem to add this. Other than that, the new comment
looks clear enough to me and does not require a bug.

+  return true;
+}
+
 PasswordModelWorker::~PasswordModelWorker() {}
 
 void PasswordModelWorker::RequestStop() {