IDL compiler: [CustomLegacyCall] interfaces, cleanup header conditional spacing

[CustomLegacyCall] is a simple extended attribute.
Also cleaned up the last ad hoc conditional handling in Jinja templates;
now it's a consistent {% filter %}.

BUG=239771

Review URL: https://codereview.chromium.org/80153003

git-svn-id: svn://svn.chromium.org/blink/trunk@162454 bbb929c8-8fbe-4397-9dbb-9b2b20218538