When serializing accessibility tree, skip invalid children.

See bug for specific repro in the wild, but essentially we need to check
if the child is valid just before serializing, and not trust the list of
children of a node.

BUG=479743

Review URL: https://codereview.chromium.org/1144363004

Cr-Commit-Position: refs/heads/master@{#332748}
  AXTreeSerializerTest() {}

  AXTreeSerializerTest() {}
