Bug 1848909: Add a11y support for XUL toolbarbuttons labelled by a text leaf. r=eeejay

Normally, XUL toolbarbutton elements are labelled using the label attribute or a child label element.
However, there are some instances such as in the Synced Tabs menu where a toolbarbutton contains only a text leaf.
Previously, these buttons exposed no label to accessibility APIs.
To fix this, we now allow a text leaf child in the accessibility tree.

We already supported this for XUL button elements.
Also, there was a bunch of existing common code in IsAcceptableChild for XULToolbarbuttonAccessible and XULButtonAccessible and the former subclasses the latter.
Therefore, XULToolbarbuttonAccessible::IsAcceptableChild has been refactored to call the base class method, adding an additional check specific to toolbarbuttons.

Differential Revision: https://phabricator.services.mozilla.com/D191713

accessible/tests/mochitest/name/test_general.xhtml

@@ -133,6 +133,7 @@
 
       // Button labelled by a text child.
       testName("button_text", "Text");
+      testName("toolbarbutton_text", "Text");
 
       // ARIA role option is presented allowing the name calculation from
       // the visible children (bug 443081)
@@ -303,6 +304,7 @@
   <!-- name from children -->
   <box id="box_children" role="button">14</box>
   <button id="button_text">Text</button>
+  <toolbarbutton id="toolbarbutton_text">Text</toolbarbutton>
 
   <!-- name from children, hidden children -->
   <vbox role="listbox" tabindex="0">

accessible/xul/XULFormControlAccessible.cpp

@@ -406,12 +406,10 @@ bool XULToolbarButtonAccessible::IsSeparator(LocalAccessible* aAccessible) {
 // XULToolbarButtonAccessible: Widgets
 
 bool XULToolbarButtonAccessible::IsAcceptableChild(nsIContent* aEl) const {
-  // In general XUL button has not accessible children. Nevertheless menu
-  // buttons can have popup accessibles (@type="menu" or columnpicker).
-  // Also: Toolbar buttons can have labels as children.
-  // But only if the label attribute is not present.
-  return aEl->IsXULElement(nsGkAtoms::menupopup) ||
-         aEl->IsXULElement(nsGkAtoms::popup) ||
+  return XULButtonAccessible::IsAcceptableChild(aEl) ||
+         // In addition to the children allowed by buttons, toolbarbuttons can
+         // have labels as children, but only if the label attribute is not
+         // present.
          (aEl->IsXULElement(nsGkAtoms::label) &&
           !mContent->AsElement()->HasAttr(nsGkAtoms::label));
 }