fix: restore accessibility window title on macOS (#21466)

Electron's `AtomNSWindow` implements `accessibilityAttributeValue` to
provide various accessibility info to the OS, including window titles.

Chromium 75 changed to Apple's newer accessibility API for window titles
in the super class that `AtomNSWindow` inherits from. macOS still
supports both the old and new style APIs, but it will prefer the new
style if it is implemented.  This means the Electron window title is
being ignored because the newer API at the Chromium level has taken
precedence.

By implementing the newer accessibility API in `AtomNSWindow`, this
restores correct accessibility window titles in macOS Electron apps.

This is a regression has been present since Electron 6.0.0 (the first
release including the Chromium change above).

shell/browser/ui/cocoa/atom_ns_window.mm

@@ -103,8 +103,6 @@ - (void)setFrame:(NSRect)windowFrame display:(BOOL)displayViews {
 }
 
 - (id)accessibilityAttributeValue:(NSString*)attribute {
-  if ([attribute isEqual:NSAccessibilityTitleAttribute])
-    return base::SysUTF8ToNSString(shell_->GetTitle());
   if ([attribute isEqual:NSAccessibilityEnabledAttribute])
     return [NSNumber numberWithBool:YES];
   if (![attribute isEqualToString:@"AXChildren"])
@@ -125,6 +123,10 @@ - (id)accessibilityAttributeValue:(NSString*)attribute {
   return [children filteredArrayUsingPredicate:predicate];
 }
 
+- (NSString*)accessibilityTitle {
+  return base::SysUTF8ToNSString(shell_->GetTitle());
+}
+
 - (BOOL)canBecomeMainWindow {
   return !self.disableKeyOrMainWindow;
 }