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* Add back the `from` and `to` attributes in the IQ stanza, and validate
  them on reception rather than blindly setting them, as discussed
  offline.
* Remove the use of XmppStreamParser since XmlElement already has a
  ForStr() method.

Bug: 962765
Change-Id: I730f352bff15c9d0c2adea7504f67365c491b4a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1623485
Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662274}