IDL compiler: [CustomIsReachable] interfaces

One use: Nodelist.idl

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/78263004

git-svn-id: svn://svn.chromium.org/blink/trunk@162448 bbb929c8-8fbe-4397-9dbb-9b2b20218538