[sync] Componentize sync: Part 2: Add SYNC_EXPORTs to files in src/sync/

sync/js/js_event_handler.h

-// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SYNC_JS_JS_EVENT_HANDLER_H_
 #define SYNC_JS_JS_EVENT_HANDLER_H_
 
 // See README.js for design comments.
 
 #include <string>
 
+#include "sync/base/sync_export.h"
+
 namespace syncer {
 
 class JsEventDetails;
 
 // An interface for objects that handle Javascript events (e.g.,
 // WebUIs).
-class JsEventHandler {
+class SYNC_EXPORT JsEventHandler {
  public:
   virtual void HandleJsEvent(
       const std::string& name, const JsEventDetails& details) = 0;
 
  protected:
   virtual ~JsEventHandler() {}
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_JS_JS_EVENT_HANDLER_H_