Bug 1617312: Ensure labelling attributes return nil for text leaves. r=eeejay

Differential Revision: https://phabricator.services.mozilla.com/D66323

--HG--
extra : moz-landing-system : lando

accessible/mac/mozTextAccessible.mm

@@ -565,6 +565,9 @@ inline NSString* ToNSString(id aValue) {
   if (!supportedAttributes) {
     supportedAttributes = [[super accessibilityAttributeNames] mutableCopy];
     [supportedAttributes removeObject:NSAccessibilityChildrenAttribute];
+    // We remove our AXTitleUIElement here to avoid an IPC call in the
+    // parent class when locating values for our attributes.
+    [supportedAttributes removeObject:NSAccessibilityTitleUIElementAttribute];
   }
 
   return supportedAttributes;
@@ -606,4 +609,8 @@ inline NSString* ToNSString(id aValue) {
   return 0;
 }
 
+- (NSString*)accessibilityLabel {
+  return nil;
+}
+
 @end