IDL: Remove FIXMEs in interface.cpp template

Just for cleanup. No changes in behavior.
- Rename some blocks
- Remove incorrect FIXME

BUG=345503

Review URL: https://codereview.chromium.org/364803002

git-svn-id: svn://svn.chromium.org/blink/trunk@177494 bbb929c8-8fbe-4397-9dbb-9b2b20218538