DOM: Observable EventTarget integration 1/N

[chromium-wpt-export-bot]

This CL implements "limited" and "leaky" EventTarget integration with
the Observable API. See below for what "limited" and "leaky" mean.
Concretely, this involves introducing the on() method to the
EventTarget interface, so that all EventTargets can return Observables
that listen for events. This is the part that really makes Observables a
"better addEventListener()".

This is the first instance of a natively-constructed Observable, as
opposed to a JS-constructed Observable. This means the subscription
callback passed to the Observable constructor is not just a JS callback
function with user-defined code, but instead is a C++ delegate class,
called SubscribeDelegate which has its first concrete implementation
provided by EventTarget (in event_target.cc). The concrete
implementation of this interface that this CL introduces, adds an event
listener to the given EventTarget, upon subscription. The events are
forwarded to the Subscriber's next() method. This is what unlocks
more ergonomic event handling with the composable Observable primitive
and all of its (coming) operators.

1. The EventTarget integration is considered "limited" because we do not
   support any of the AddEventListenerOptions yet, as of this CL. A
   subsequent CL will add support for a more restricted version of the
   AddEventListenerOptions, called ObservableEventListenerOptions,
   which does not include a once option, or an AbortSignal, since
   Observable operators and subscription is responsible for managing those
   aspects. Concretely, an ObservableEventListenerOptions will
   resolve to an AddEventListenerOptionsResolved accordingly. See:

• Use a restricted version of AddEventListenerOptions? WICG/observable#66
• Use ObservableEventListenerOptions WICG/observable#67
• Way for single event subscription? WICG/observable#65

2. The EventTarget integration is considered "leaky" as of this CL,
   because there is currently no way to remove an event listener added by
   an EventTarget-vended Observable. This will come in a subsequent CL,
   which will pass the test that is currently failing in this CL. See
   How to removeEventListener? WICG/observable#75 for discussion about
   tying the subscription termination to removing an event listener.
   From a technical perspective, this is pretty easy — it involves adding
   an abort algorithm to Subscriber#signal (which has already been wired
   up properly by now!) that removes the given per-Subscription
   ObservableEventListener NativeEventListener from the associated
   EventTarget. That implementation has already been sketched out in
   https://crrev.com/c/4262153 and the design doc. It will included in a
   follow-up CL, to reduce the complexity of this one.

For WPTs:
Co-authored-by: [email protected]

R=[email protected]

Bug: 1485981
Change-Id: Iafeddb0894b8eed2be1d95c181fc44d7650c0d47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5073394
Reviewed-by: Mason Freed <[email protected]>
Commit-Queue: Dominic Farolino <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1237501}

[wpt-pr-bot]

The review process for this patch is being conducted in the Chromium project.