Auto merge of #23249 - georgeroman:implement_mouseevent_buttons, r=Manishearth

Implement MouseEvent buttons attribute

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #22884

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tests/wpt/metadata/dom/events/Event-subclasses-constructors.html.ini

@@ -1,20 +1,5 @@
 [Event-subclasses-constructors.html]
   type: testharness
-  [MouseEvent constructor (no argument)]
-    expected: FAIL
-
-  [MouseEvent constructor (undefined argument)]
-    expected: FAIL
-
-  [MouseEvent constructor (null argument)]
-    expected: FAIL
-
-  [MouseEvent constructor (empty argument)]
-    expected: FAIL
-
-  [MouseEvent constructor (argument with default values)]
-    expected: FAIL
-
   [MouseEvent constructor (argument with non-default values)]
     expected: FAIL
 

tests/wpt/metadata/uievents/idlharness.window.js.ini

@@ -5,9 +5,6 @@
   [MouseEvent interface: new WheelEvent("event") must inherit property "buttons" with the proper type]
     expected: FAIL
 
-  [MouseEvent interface: new MouseEvent("event") must inherit property "buttons" with the proper type]
-    expected: FAIL
-
   [UIEvent interface: new CompositionEvent("event") must inherit property "view" with the proper type]
     expected: FAIL
 
@@ -152,9 +149,6 @@
   [WheelEvent interface: existence and properties of interface prototype object's @@unscopables property]
     expected: FAIL
 
-  [MouseEvent interface: attribute buttons]
-    expected: FAIL
-
   [MouseEvent interface: new WheelEvent("event") must inherit property "screenY" with the proper type]
     expected: FAIL