IDL compiler: add EventHandler

Add EventHandler special case.
The getter isn't too bad, though the setter will be more complicated.

(For now I've inlined the special casing, as it's short;
when we add setters we'll want to refactor this into a separate function.)

BUG=239771

Review URL: https://codereview.chromium.org/25667002

git-svn-id: svn://svn.chromium.org/blink/trunk@158691 bbb929c8-8fbe-4397-9dbb-9b2b20218538