IDL compiler: Add linebreaks to defaultSignature

This syncs Python to r161030 (so Perl/Python consistent),
and adds linebreaks for legibility in the template.
(Otherwise we get a hugely long line with if/else clauses
in the middle.)

This is a followup to:
Clean up code generator before implementing [ExposeV8GetterAndSetter]
https://codereview.chromium.org/54123003/
...and makes the Perl a bit longer (indent and linebreaks),
but makes the Python template much clearer.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/52593007

git-svn-id: svn://svn.chromium.org/blink/trunk@161034 bbb929c8-8fbe-4397-9dbb-9b2b20218538