Move IDL path handling from Code Generator to idl_reader

Cleanup now that Code Generator has landed.
This means that (other than resolving parents, which is separate issue
that I'll fix later), the Code Generator doesn't need to know anything
about IDL files or paths.

No changes to generated files (gen/webkit).

Review URL: https://chromiumcodereview.appspot.com/21087011

git-svn-id: svn://svn.chromium.org/blink/trunk@155182 bbb929c8-8fbe-4397-9dbb-9b2b20218538