[Thread Safety] Replace WebString bound by cross-thread base::Bind() with base::string16

content/child/background_sync/background_sync_provider_thread_proxy.cc

Index: content/child/background_sync/background_sync_provider_thread_proxy.cc
diff --git a/content/child/background_sync/background_sync_provider_thread_proxy.cc b/content/child/background_sync/background_sync_provider_thread_proxy.cc
index 1937cf3e0ae750773c71bc6804da22dc729c0ef6..90584bfc8380a6d31a2280ddd33303e83e52439a 100644
--- a/content/child/background_sync/background_sync_provider_thread_proxy.cc
+++ b/content/child/background_sync/background_sync_provider_thread_proxy.cc
@@ -106,8 +106,10 @@ void BackgroundSyncProviderThreadProxy::unregisterBackgroundSync(
   main_thread_task_runner_->PostTask(
       FROM_HERE,
       base::Bind(&BackgroundSyncProvider::unregisterBackgroundSync,
-                 base::Unretained(sync_provider_), periodicity, id, tag,
-                 service_worker_registration,
+                 base::Unretained(sync_provider_), periodicity, id,
+                 // We cast WebString to string16 before crossing threads
+                 // for thread-safety.
+                 static_cast<base::string16>(tag), service_worker_registration,
                  new CallbackThreadAdapter<bool*, blink::WebSyncError*>(
                      make_scoped_ptr(callbacks),
                      WorkerTaskRunner::Instance()->CurrentWorkerId())));
@@ -123,8 +125,10 @@ void BackgroundSyncProviderThreadProxy::getRegistration(
   main_thread_task_runner_->PostTask(
       FROM_HERE,
       base::Bind(&BackgroundSyncProvider::getRegistration,
-                 base::Unretained(sync_provider_), periodicity, tag,
-                 service_worker_registration,
+                 base::Unretained(sync_provider_), periodicity,
+                 // We cast WebString to string16 before crossing threads
+                 // for thread-safety.
+                 static_cast<base::string16>(tag), service_worker_registration,
                  new CallbackThreadAdapter<blink::WebSyncRegistration*,
                                            blink::WebSyncError*>(
                      make_scoped_ptr(callbacks),