Disable tab syncing when history saving is disabled.

chrome/browser/sync/glue/non_frontend_data_type_controller.cc

Index: chrome/browser/sync/glue/non_frontend_data_type_controller.cc
diff --git a/chrome/browser/sync/glue/non_frontend_data_type_controller.cc b/chrome/browser/sync/glue/non_frontend_data_type_controller.cc
index ee7819cad224d1af38d995f882ff5aaa290e2686..e20c3cc9df3a14e3993d9a01af1a0ab764f078c2 100644
--- a/chrome/browser/sync/glue/non_frontend_data_type_controller.cc
+++ b/chrome/browser/sync/glue/non_frontend_data_type_controller.cc
@@ -292,7 +292,6 @@ sync_driver::DataTypeController::State NonFrontendDataTypeController::state()
 
 void NonFrontendDataTypeController::OnSingleDataTypeUnrecoverableError(
     const syncer::SyncError& error) {
-  DCHECK(IsOnBackendThread());

[Andrew T Wilson (Slow), 2014/08/21 14:10:37]

Nicolas there are a few things we noticed here:

1) TypedUrlDataTypeController is a NonFrontendDataTypeController. However, if
the history service gets disabled, *there is no more backend thread*, but we
need to generate an unrecoverable error. The only thread we can do this on is
the UI thread, but it hits this DCHECK.

We can either remove this DCHECK(), or else we can call
RecordUnrecoverableError() and DisableImpl() directly from
TypedUrlDataTypeController(). What is the right way to go?

2) It looks like RecordUnrecoverableError() can be called from any thread,
because it's called both from here (backend thread) and also from StartDone().
This means that the error_callback_ can be invoked from any thread. Is this
correct/intended?

[Nicolas Zea, 2014/08/26 17:59:30]

I think you should be fine just calling DisableImpl. RecordUnrecoverableError is
just for UMA tracking. Theoretically (last I checked) a specific UMA histogram
callsite is not thread safe (it uses some weird static variables under the
hood), so I'd just avoid calling it altogether for policy errors. Plus, they're
errors by design, rather than bugs/unknown issues.

[Joao da Silva, 2014/08/26 18:43:02]

Thanks for clarifying, done.

Unrelated to this CL: note that RecordUnrecoverableError is called from 2
places:

- OnSingleDataTypeUnrecoverableError, on the backend thread
- StartDone, on the UI thread

And then RecordUnrecoverableError invokes the error_callback_ on either of those
threads; shouldn't the callback be invoked in the same thread in both cases?

   DCHECK_EQ(type(), error.model_type());
   RecordUnrecoverableError(error.location(), error.message());
   BrowserThread::PostTask(BrowserThread::UI, error.location(),