Implement non-XR Gamepad discovery and input (#31200)

* Create embedder event to send to constellation

* Handle gamepad message in constellation, send to script thread

* Handle GamepadEvent in script thread and dispatch event to document

* Add missing Clones, fix event

* Add gamepad task source

* Adjust GamepadIndex type, remove unused imports

* Add internal getter for gamepads list

* Update gamepad new methods

* Handle gamepad connect and disconnect events

* Proto will be none, no need for HandleObject

* Initialize buttons and axes to standard mapping

* Adjust update type index types

* Update GamepadButton update function

* Adjust Gamepad mapping comments to match spec, add update logic

* Amend comment

* Update button and axis inputs on Updated event

* Add GilRs as gamepad backend in servoshell

* Add spec links, queue gamepad updates on task source

* ./mach fmt

* Fix comment length

* Split out button init, update spec comments

* Move gamepad event handling from document to global

* Map and normalize axes/button values

* Use std::time for gamepad timestamp

* Adjust gamepad handling in event loop

* Move button press/touch check into map+normalize function

- Small change but is more in line with spec

* ./mach fmt

* Update comment spec links and warning messages

* Doc comments -> regular comments

* Add window event handlers for gamepad connect/disconnect

* Adjust gamepad disconnect behavior

* Add missing TODO's, adjust gamepad/gamepadbutton list methods and formatting

* Update button handling from gilrs, add comments

* Enable gamepad pref during WPT tests and update expectations

* Update WPT expectations in meta-legacy-layout

components/shared/script/lib.rs

@@ -569,6 +569,8 @@ pub enum CompositorEvent {
     CompositionEvent(CompositionEvent),
     /// Virtual keyboard was dismissed
     IMEDismissedEvent,
+    /// Connected gamepad state updated
+    GamepadEvent(GamepadEvent),
 }
 
 impl From<&CompositorEvent> for CompositorEventVariant {
@@ -582,6 +584,7 @@ impl From<&CompositorEvent> for CompositorEventVariant {
             CompositorEvent::KeyboardEvent(..) => CompositorEventVariant::KeyboardEvent,
             CompositorEvent::CompositionEvent(..) => CompositorEventVariant::CompositionEvent,
             CompositorEvent::IMEDismissedEvent => CompositorEventVariant::IMEDismissedEvent,
+            CompositorEvent::GamepadEvent(..) => CompositorEventVariant::GamepadEvent,
         }
     }
 }
@@ -1327,3 +1330,43 @@ impl SerializedImageData {
         }
     }
 }
+
+#[derive(
+    Clone, Copy, Debug, Deserialize, Eq, Hash, MallocSizeOf, Ord, PartialEq, PartialOrd, Serialize,
+)]
+/// Index of gamepad in list of system's connected gamepads
+pub struct GamepadIndex(pub usize);
+
+#[derive(Clone, Debug, Deserialize, Serialize)]
+/// The minimum and maximum values that can be reported for axis or button input from this gamepad
+pub struct GamepadInputBounds {
+    /// Minimum and maximum axis values
+    pub axis_bounds: (f64, f64),
+    /// Minimum and maximum button values
+    pub button_bounds: (f64, f64),
+}
+
+#[derive(Clone, Debug, Deserialize, Serialize)]
+/// The type of Gamepad event
+pub enum GamepadEvent {
+    /// A new gamepad has been connected
+    /// <https://www.w3.org/TR/gamepad/#event-gamepadconnected>
+    Connected(GamepadIndex, String, GamepadInputBounds),
+    /// An existing gamepad has been disconnected
+    /// <https://www.w3.org/TR/gamepad/#event-gamepaddisconnected>
+    Disconnected(GamepadIndex),
+    /// An existing gamepad has been updated
+    /// <https://www.w3.org/TR/gamepad/#receiving-inputs>
+    Updated(GamepadIndex, GamepadUpdateType),
+}
+
+#[derive(Clone, Debug, Deserialize, Serialize)]
+/// The type of Gamepad input being updated
+pub enum GamepadUpdateType {
+    /// Axis index and input value
+    /// <https://www.w3.org/TR/gamepad/#dfn-represents-a-standard-gamepad-axis>
+    Axis(usize, f64),
+    /// Button index and input value
+    /// <https://www.w3.org/TR/gamepad/#dfn-represents-a-standard-gamepad-button
+    Button(usize, f64),
+}