Blink-in-JS: A very simple refactoring about a way to get a basename of an IDL file

Just a clean-up of idl_reader.py.

BUG=341031

Review URL: https://codereview.chromium.org/469253004

git-svn-id: svn://svn.chromium.org/blink/trunk@180333 bbb929c8-8fbe-4397-9dbb-9b2b20218538