Default navigation in VoiceOver includes only visible elements

Navigation with the VO cursor should not visit hidden elements.
This had regressed because we started including hidden elements
in the accessible object hierarchy on purpose, so that when
a page accidentally overused aria-hidden, that users would
still hear something useful when tabbing to focusable items.
With this CL, users will not generally be able to accidentally
navigate to aria-hidden items, but if they manage to actually
focus one with the keyboard, it will still announce what it is.

Bug: 982408
Change-Id: I19d5fe4947e5fb1c7493f0e67d7fe8e13d9a255e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761717Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689677}

content/browser/accessibility/browser_accessibility_cocoa.mm

@@ -559,7 +559,7 @@ bool InitializeAccessibilityTreeSearch(OneShotAccessibilityTreeSearch* search,
   if ([immediateDescendantsOnlyParameter isKindOfClass:[NSNumber class]])
     immediateDescendantsOnly = [immediateDescendantsOnlyParameter boolValue];
 
-  bool visibleOnly = false;
+  bool visibleOnly = true;  // Default to visible only.
   NSNumber* visibleOnlyParameter = [dictionary objectForKey:@"AXVisibleOnly"];
   if ([visibleOnlyParameter isKindOfClass:[NSNumber class]])
     visibleOnly = [visibleOnlyParameter boolValue];