IDL compiler: [InitializedByEventConstructor] attribute any

Pretty complex special case.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/110023002

git-svn-id: svn://svn.chromium.org/blink/trunk@163416 bbb929c8-8fbe-4397-9dbb-9b2b20218538