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/constellation/constellation.rs

@@ -142,7 +142,7 @@ use script_traits::CompositorEvent::{MouseButtonEvent, MouseMoveEvent};
 use script_traits::{
     webdriver_msg, AnimationState, AnimationTickType, AuxiliaryBrowsingContextLoadInfo,
     BroadcastMsg, CompositorEvent, ConstellationControlMsg, DiscardBrowsingContext,
-    DocumentActivity, DocumentState, HistoryEntryReplacement, IFrameLoadInfo,
+    DocumentActivity, DocumentState, GamepadEvent, HistoryEntryReplacement, IFrameLoadInfo,
     IFrameLoadInfoWithData, IFrameSandboxState, IFrameSizeMsg, Job, LayoutControlMsg,
     LayoutMsg as FromLayoutMsg, LoadData, LoadOrigin, LogEntry, MediaSessionActionType,
     MessagePortMsg, MouseEventType, PortMessageTask, SWManagerMsg, SWManagerSenders,
@@ -1550,6 +1550,9 @@ where
                     EmbedderMsg::ReadyToPresent,
                 ));
             },
+            FromCompositorMsg::Gamepad(gamepad_event) => {
+                self.handle_gamepad_msg(gamepad_event);
+            },
         }
     }
 
@@ -5466,4 +5469,40 @@ where
             error!("Got a media session action but no active media session is registered");
         }
     }
+
+    /// Handle GamepadEvents from the embedder and forward them to the script thread
+    fn handle_gamepad_msg(&mut self, event: GamepadEvent) {
+        // Send to the focused browsing contexts' current pipeline.
+        let focused_browsing_context_id = self
+            .webviews
+            .focused_webview()
+            .map(|(_, webview)| webview.focused_browsing_context_id);
+        match focused_browsing_context_id {
+            Some(browsing_context_id) => {
+                let event = CompositorEvent::GamepadEvent(event);
+                let pipeline_id = match self.browsing_contexts.get(&browsing_context_id) {
+                    Some(ctx) => ctx.pipeline_id,
+                    None => {
+                        return warn!(
+                            "{}: Got gamepad event for nonexistent browsing context",
+                            browsing_context_id,
+                        );
+                    },
+                };
+                let msg = ConstellationControlMsg::SendEvent(pipeline_id, event);
+                let result = match self.pipelines.get(&pipeline_id) {
+                    Some(pipeline) => pipeline.event_loop.send(msg),
+                    None => {
+                        return debug!("{}: Got gamepad event after closure", pipeline_id);
+                    },
+                };
+                if let Err(e) = result {
+                    self.handle_send_error(pipeline_id, e);
+                }
+            },
+            None => {
+                warn!("No focused webview to handle gamepad event");
+            },
+        }
+    }
 }