Pref service: enable for user prefs in chrome behind a flag.

services/preferences/public/cpp/persistent_pref_store_client.cc

Index: services/preferences/public/cpp/persistent_pref_store_client.cc
diff --git a/services/preferences/public/cpp/persistent_pref_store_client.cc b/services/preferences/public/cpp/persistent_pref_store_client.cc
index 032e081625427f2e1bf3687f18e6252a2da37960..74c9b116fbd7d9bae66fe0bc950077ad0f9e36bc 100644
--- a/services/preferences/public/cpp/persistent_pref_store_client.cc
+++ b/services/preferences/public/cpp/persistent_pref_store_client.cc
@@ -6,7 +6,9 @@
 
 #include <utility>
 
+#include "base/threading/thread_restrictions.h"
 #include "base/values.h"
+#include "mojo/public/cpp/bindings/sync_call_restrictions.h"
 
 namespace prefs {
 
@@ -70,6 +72,8 @@ PersistentPrefStore::PrefReadError PersistentPrefStoreClient::ReadPrefs() {
   std::unique_ptr<base::DictionaryValue> local_prefs;
   mojom::PersistentPrefStorePtr pref_store;
   mojom::PrefStoreObserverRequest observer_request;
+  base::ThreadRestrictions::AssertWaitAllowed();

[jam, 2017/03/22 14:58:12]

nit: curious why you added this? mojo::SyncCallRestrictions already has a
ScopedAllowWait member. I think no need to worry about both restrictions, just
the mojo one.

[Sam McNally, 2017/03/28 23:06:22]

It was intended to detect inadvertent post-startup sync pref creation. Removed
it.

+  mojo::SyncCallRestrictions::ScopedAllowSyncCall allow_sync_calls;
   if (!connector_->Connect(&read_error, &read_only, &local_prefs, &pref_store,
                            &observer_request)) {
     NOTREACHED();