IDL compiler: Optional callback interface arguments

(42 uses. *42*!)

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/127793002

git-svn-id: svn://svn.chromium.org/blink/trunk@164733 bbb929c8-8fbe-4397-9dbb-9b2b20218538