[Sync] Pass a TaskRunner into SBHI/SBHC, not a thread or message loop.

components/sync/driver/glue/sync_backend_host_core.h

Index: components/sync/driver/glue/sync_backend_host_core.h
diff --git a/components/sync/driver/glue/sync_backend_host_core.h b/components/sync/driver/glue/sync_backend_host_core.h
index 129c63d3293ba2c47056e0105e43a662c3bf99ba..3737f513cc98ee658b0eef09f3d88c9a022c0dcf 100644
--- a/components/sync/driver/glue/sync_backend_host_core.h
+++ b/components/sync/driver/glue/sync_backend_host_core.h
@@ -14,6 +14,7 @@
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/single_thread_task_runner.h"
 #include "base/timer/timer.h"
 #include "base/trace_event/memory_dump_provider.h"
 #include "components/invalidation/public/invalidation.h"
@@ -32,7 +33,7 @@ class SyncBackendHostImpl;
 // Utility struct for holding initialization options.
 struct DoInitializeOptions {
   DoInitializeOptions(
-      base::MessageLoop* sync_loop,
+      scoped_refptr<base::SingleThreadTaskRunner> sync_task_runner,
       SyncBackendRegistrar* registrar,
       const std::vector<scoped_refptr<ModelSafeWorker>>& workers,
       const scoped_refptr<ExtensionsActivity>& extensions_activity,
@@ -55,7 +56,7 @@ struct DoInitializeOptions {
       const std::map<ModelType, int64_t>& invalidation_versions);
   ~DoInitializeOptions();
 
-  base::MessageLoop* sync_loop;
+  scoped_refptr<base::SingleThreadTaskRunner> sync_task_runner;
   SyncBackendRegistrar* registrar;
   std::vector<scoped_refptr<ModelSafeWorker>> workers;
   scoped_refptr<ExtensionsActivity> extensions_activity;
@@ -277,9 +278,8 @@ class SyncBackendHostCore
   // Our parent SyncBackendHost.
   WeakHandle<SyncBackendHostImpl> host_;
 
-  // The loop where all the sync backend operations happen.
-  // Non-null only between calls to DoInitialize() and ~Core().
-  base::MessageLoop* sync_loop_;
+  // The task runner where all the sync engine operations happen.
+  scoped_refptr<base::SingleThreadTaskRunner> sync_task_runner_;
 
   // Our parent's registrar (not owned).  Non-null only between
   // calls to DoInitialize() and DoShutdown().