IDL compiler: EventTarget arguments

12 => 11

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/155523003

git-svn-id: svn://svn.chromium.org/blink/trunk@166455 bbb929c8-8fbe-4397-9dbb-9b2b20218538