File numbers were added to the diff view in c5069b1cd2, but would not
appear because the binding passed the file number whereas the visibility
method expected a file path. With this change, the method is rewritten
to accept the file number and a JSDoc comment is added to prevent the
type mismatch. The CSS is also modified to hide the element until the
dependent properties have evaluated.
Bug: Issue 4917
Change-Id: I7b880e65612e3b75e80b66dfd7a258608199c49b
54ce5dc8e3