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

chrome/browser/sync/backend_unrecoverable_error_handler.h

Index: chrome/browser/sync/backend_unrecoverable_error_handler.h
diff --git a/chrome/browser/sync/backend_unrecoverable_error_handler.h b/chrome/browser/sync/backend_unrecoverable_error_handler.h
index 872cef4f9ab0f2058ac3e8b78c9a322a2c9295d9..b5823ff54de0af7d169873c08faab4deb5fdd23c 100644
--- a/chrome/browser/sync/backend_unrecoverable_error_handler.h
+++ b/chrome/browser/sync/backend_unrecoverable_error_handler.h
@@ -19,7 +19,7 @@ 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,
@@ -30,4 +30,3 @@ class BackendUnrecoverableErrorHandler
 };
 }  // namespace browser_sync
 #endif  // CHROME_BROWSER_SYNC_BACKEND_UNRECOVERABLE_ERROR_HANDLER_H_
-