Add missing (and remove superfluous) 'explicit' from constructors.

chrome/browser/sync/backend_unrecoverable_error_handler.h

 // Copyright (c) 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 CHROME_BROWSER_SYNC_BACKEND_UNRECOVERABLE_ERROR_HANDLER_H_
 #define CHROME_BROWSER_SYNC_BACKEND_UNRECOVERABLE_ERROR_HANDLER_H_
 
 #include <string>
 
 #include "base/location.h"
 #include "base/memory/weak_ptr.h"
 
 #include "sync/internal_api/public/util/unrecoverable_error_handler.h"
 #include "sync/internal_api/public/util/weak_handle.h"
 
 class ProfileSyncService;
 namespace browser_sync {
 
 class BackendUnrecoverableErrorHandler
     : public syncer::UnrecoverableErrorHandler {
  public:
-  BackendUnrecoverableErrorHandler(
+  explicit BackendUnrecoverableErrorHandler(
       const syncer::WeakHandle<ProfileSyncService>& service);
   virtual ~BackendUnrecoverableErrorHandler();
   virtual void OnUnrecoverableError(const tracked_objects::Location& from_here,
                                     const std::string& message) OVERRIDE;
 
  private:
   syncer::WeakHandle<ProfileSyncService> service_;
 };
 }  // namespace browser_sync
 #endif  // CHROME_BROWSER_SYNC_BACKEND_UNRECOVERABLE_ERROR_HANDLER_H_
-