IDL compiler: [InitializedByEventConstructor] (basic)

Lotta special cases for this too, to do in followups.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/101533007

git-svn-id: svn://svn.chromium.org/blink/trunk@163402 bbb929c8-8fbe-4397-9dbb-9b2b20218538