Make code slightly less complicated.

idl_files_list contains the contents of idl_files, so `cat <(idl_files_list)`
will always evaluate to idl_files. No need to shell out to cat for this.

No functionality change. This was added in
https://bugs.webkit.org/show_bug.cgi?id=73394

BUG=none
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/26980003

git-svn-id: svn://svn.chromium.org/blink/trunk@159558 bbb929c8-8fbe-4397-9dbb-9b2b20218538