IDL compiler: [CustomWrap] interfaces

(Was [CustomToV8], renamed.)

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/79173009

git-svn-id: svn://svn.chromium.org/blink/trunk@162512 bbb929c8-8fbe-4397-9dbb-9b2b20218538