Add togglePopover() method for popovers

Per the resolution [1] there's a desire to add a togglePopover()
convenience method. This CL adds that method plus tests.

[1] openui/open-ui#610 (comment)

Bug: 1307772
Change-Id: Icd39c766786e7d1f6475a86ca8a7a1e89b66182e

html/semantics/popovers/popover-attribute-basic.tentative.html

@@ -79,9 +79,15 @@
     popover.hidePopover();
     assertPopoverVisibility(popover, /*isPopover*/true, /*expectedVisibility*/false, 'After hidePopover(), a popover should be hidden');
     assert_throws_dom("InvalidStateError",() => popover.hidePopover(),'Calling hidePopover on a hidden popover should throw InvalidStateError');
+    popover.togglePopover();
+    assertPopoverVisibility(popover, /*isPopover*/true, /*expectedVisibility*/true, 'After togglePopover() on hidden popover, it should be visible');
+    popover.togglePopover();
+    assertPopoverVisibility(popover, /*isPopover*/true, /*expectedVisibility*/false, 'After togglePopover() on visible popover, it should be hidden');
     const parent = popover.parentElement;
     popover.remove();
     assert_throws_dom("InvalidStateError",() => popover.showPopover(),'Calling showPopover on a disconnected popover should throw InvalidStateError');
+    assert_throws_dom("InvalidStateError",() => popover.hidePopover(),'Calling hidePopover on a disconnected popover should throw InvalidStateError');
+    assert_throws_dom("InvalidStateError",() => popover.togglePopover(),'Calling hidePopover on a disconnected popover should throw InvalidStateError');
     parent.appendChild(popover);
   }
   function assertNotAPopover(nonPopover) {
@@ -93,6 +99,8 @@
     assertPopoverVisibility(nonPopover, /*isPopover*/false, expectVisible, 'Calling showPopover on a non-popover should leave it visible');
     assert_throws_dom("NotSupportedError",() => nonPopover.hidePopover(),'Calling hidePopover on a non-popover should throw NotSupportedError');
     assertPopoverVisibility(nonPopover, /*isPopover*/false, expectVisible, 'Calling hidePopover on a non-popover should leave it visible');
+    assert_throws_dom("NotSupportedError",() => nonPopover.togglePopover(),'Calling togglePopover on a non-popover should throw NotSupportedError');
+    assertPopoverVisibility(nonPopover, /*isPopover*/false, expectVisible, 'Calling togglePopover on a non-popover should leave it visible');
   }
 
   // Start with the provided examples: