[w3c/gamepad] Trigger HTML activation (PR #230)

Closes #124

This combines gamepad interaction with HTML's existing sticky/transient activation (https://html.spec.whatwg.org/#tracking-user-activation) model, per @marcoscaceres's review on #107 ("we probably want to layer this on top of HTML's 'triggered by user activation'") and this issue's suggestion to use the sticky-activation model directly, rather than defining a new gamepad-local activation concept as #107 attempted (that PR duplicated the existing spec's "contains a gamepad user gesture" definition under a colliding name and never reached HTML's real activation state -- I suggest closing it in favor of this PR).

This also matches existing shipped behavior: Chromium calls LocalFrame::NotifyUserActivation(kInteraction) from NavigatorGamepad::Gamepads() whenever GamepadComparisons::Compare() detects a qualifying button/axis transition on a visible page -- the same primitive mouse/keyboard input uses. This PR gives that behavior normative text, reusing the spec's existing "contains a gamepad user gesture" and fully active descendant of a top-level traversable with user attention conditions as the trigger, and HTML's activationnotification algorithm as the effect.

Note this extends HTML's own introduction of activation notification ("when a user interaction causes firing of an activation triggering input event... the user agent must perform the following steps before dispatching the event") — gamepad state changes aren't dispatched as that kind of event; they're observed by polling. I'm invoking the algorithm as a reusable procedure rather than a hard prerequisite check, consistent with how Chromium implements it (it calls the underlying primitive directly, not by synthesizing a fake input event). Flagging this for HTML editors in case they'd prefer activation-triggering-input-event itself be broadened instead of allowing direct invocation.  I can change to that if requested.

Deliberately out of scope: the separate, older "gamepadconnected event" section (§17) has its own partially-duplicated algorithm with stale prose ("SHOULD be dispatched when the user presses a button or moves an axis") that looks like leftover text update gamepad state was meant to supersede. Left untouched here to keep this PR reviewable; worth a separate cleanup PR.

The following tasks have been completed:

 * [x] Modified Web platform tests (https://bugzilla.mozilla.org/show_bug.cgi?id=2053976)

Implementation commitment:

 * [ ] WebKit
 * [x] Chromium (already implemented)
 * [x] Gecko (tracked in https://bugzilla.mozilla.org/show_bug.cgi?id=1679817 and https://bugzilla.mozilla.org/show_bug.cgi?id=1740573)


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  https://github.com/w3c/gamepad/pull/230

-- Commit Summary --

  * Trigger HTML activation

-- File Changes --

    M index.html (19)

-- Patch Links --

https://github.com/w3c/gamepad/pull/230.patch
https://github.com/w3c/gamepad/pull/230.diff

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