ElementKind.FILE will be used in search.findElementReferences
1. In the 'element' field of responce when a URI is at the given offset.
2. In the only element of the 'path' field of SearchResult(s) for URI references to a URI or a library identifier.
R=brianwilkerson@google.com
BUG=
Review URL: https://codereview.chromium.org/1409333002 .