IDL compiler: [EventConstructor] (basic)

Initial CL for [EventConstructor];
many additional cases to consider, to do in followups.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/106103005

git-svn-id: svn://svn.chromium.org/blink/trunk@163397 bbb929c8-8fbe-4397-9dbb-9b2b20218538