IDL compiler: EventTarget as parent

Support for event targets (used also in named constructors, hence why
I'm doing it now).

Note that this only supports EventTarget itself or immediate children,
as that's all you can determine from parsing a single IDL file.

Supporting more distant descendants efficiently requires hacking on
compute_dependencies to compute the ancestor chain, so I'll do this
when rewriting dependency resolution.

(Perl doesn't do this (it parses all the ancestors every time),
 which is one reason that it's slow.)

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/116253002

git-svn-id: svn://svn.chromium.org/blink/trunk@163947 bbb929c8-8fbe-4397-9dbb-9b2b20218538