Fix crash when system retains an accessibility object.

This is a follow-up to r207709.

BUG=242944

Review URL: https://codereview.chromium.org/58993002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233506 0039d316-1c4b-4281-b951-d872f2087c98

content/browser/accessibility/browser_accessibility_cocoa.mm

@@ -1421,6 +1421,8 @@ NSDictionary* attributeToMethodNameMap = nil;
 
   BrowserAccessibilityCocoa* hit = self;
   for (BrowserAccessibilityCocoa* child in [self children]) {
+    if (!child->browserAccessibility_)
+      continue;
     NSPoint origin = [child origin];
     NSSize size = [[child size] sizeValue];
     NSRect rect;