fix(menu): use narrower value for aria-haspopup on trigger element

Currently we set `aria-haspopup` correctly on the menu trigger, however [according to the spec](https://www.w3.org/TR/wai-aria-1.1/#aria-haspopup) we can narrow the value down so that it indicates what kind of popup will be opened.

Author: crisbeto

src/material/menu/menu-trigger.ts

@@ -73,7 +73,7 @@ const passiveEventListenerOptions = normalizePassiveListenerOptions({passive: tr
 @Directive({
   selector: `[mat-menu-trigger-for], [matMenuTriggerFor]`,
   host: {
-    'aria-haspopup': 'true',
+    'aria-haspopup': 'menu',
     '[attr.aria-expanded]': 'menuOpen || null',
     '(mousedown)': '_handleMousedown($event)',
     '(keydown)': '_handleKeydown($event)',

src/material/menu/menu.spec.ts

@@ -114,6 +114,14 @@ describe('MatMenu', () => {
     expect(overlayContainerElement.querySelector('.cdk-overlay-backdrop')).toBeFalsy();
   }));
 
+  it('should set the correct aria-haspopup value on the trigger element', fakeAsync(() => {
+    const fixture = createComponent(SimpleMenu, [], [FakeIcon]);
+    fixture.detectChanges();
+    const triggerElement = fixture.componentInstance.triggerEl.nativeElement;
+
+    expect(triggerElement.getAttribute('aria-haspopup')).toBe('menu');
+  }));
+
   it('should be able to remove the backdrop on repeat openings', fakeAsync(() => {
     const fixture = createComponent(SimpleMenu, [], [FakeIcon]);
     fixture.detectChanges();