Fix tonic include guards

BUG=
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1247843002 .
diff --git a/tonic/dart_value.h b/tonic/dart_value.h
index 9c20b67..8e8c629 100644
--- a/tonic/dart_value.h
+++ b/tonic/dart_value.h
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef SKY_ENGINE_TONIC_DART_VALUE_H_
-#define SKY_ENGINE_TONIC_DART_VALUE_H_
+#ifndef TONIC_DART_VALUE_H_
+#define TONIC_DART_VALUE_H_
 
 #include "base/logging.h"
 #include "dart/runtime/include/dart_api.h"
@@ -62,4 +62,4 @@
 
 }  // namespace tonic
 
-#endif  // SKY_ENGINE_TONIC_DART_VALUE_H_
+#endif  // TONIC_DART_VALUE_H_