IDL compiler: [Conditional] interfaces

Simple one, but various cleanup:
* cleanup existing template (use filter)
* fix condition names in tests (conditions are in ALL_CAPS)
* regularize Perl (remove irregular \n)
* move conditional handling to v8_interface.py and v8_callback_interface.py
Previously was combined in code_generator_v8.py,
which makes it hard to find and over-factored;
now it's with the others.

BUG=239771

Review URL: https://codereview.chromium.org/59243003

git-svn-id: svn://svn.chromium.org/blink/trunk@162325 bbb929c8-8fbe-4397-9dbb-9b2b20218538