Fix regression in Mac AX live region support

As part of a refactoring, the code to detect if a new live region
was added was accidentally modified to trigger if a live region
was changed in any way. It should only trigger on a brand-new
live region.

BUG=469276

Review URL: https://codereview.chromium.org/1030373003

Cr-Commit-Position: refs/heads/master@{#322434}

content/browser/accessibility/browser_accessibility_manager_mac.mm

@@ -150,6 +150,8 @@ void BrowserAccessibilityManagerMac::OnAtomicUpdateFinished(
 
   bool created_live_region = false;
   for (size_t i = 0; i < changes.size(); ++i) {
+    if (changes[i].type != NODE_CREATED && changes[i].type != SUBTREE_CREATED)
+      continue;
     BrowserAccessibility* obj = GetFromAXNode(changes[i].node);
     if (obj && obj->HasStringAttribute(ui::AX_ATTR_LIVE_STATUS)) {
       created_live_region = true;