IDL compiler: [CustomElementCallbacks]

Remaining: 23 => 17
Also comment details on the "super-compact getter", which is actually
not such a big deal AFAICT.

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/155543002

git-svn-id: svn://svn.chromium.org/blink/trunk@166444 bbb929c8-8fbe-4397-9dbb-9b2b20218538