[Sync] Replace uses of ObserverListThreadSafe with WeakHandles

chrome/browser/sync/engine/syncer_unittest.cc

Index: chrome/browser/sync/engine/syncer_unittest.cc
diff --git a/chrome/browser/sync/engine/syncer_unittest.cc b/chrome/browser/sync/engine/syncer_unittest.cc
index 998512071efefee8dcb2cba4c4fd58d75745ddad..b4b4003aaff2e0a6bdce01238739388c60c2b308 100644
--- a/chrome/browser/sync/engine/syncer_unittest.cc
+++ b/chrome/browser/sync/engine/syncer_unittest.cc
@@ -16,6 +16,7 @@
 #include "base/compiler_specific.h"
 #include "base/location.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/message_loop.h"
 #include "base/string_number_conversions.h"
 #include "base/stringprintf.h"
 #include "base/time.h"
@@ -468,6 +469,8 @@ class SyncerTest : public testing::Test,
     return GetField(metahandle, field, false);
   }
 
+  MessageLoop message_loop_;
+
   // Some ids to aid tests. Only the root one's value is specific. The rest
   // are named for test clarity.
   // TODO(chron): Get rid of these inbuilt IDs. They only make it