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This was because I was overriding XMLHttpRequest's open and send
functions a second time when reinjecting the scripts. The fix is to
ensure those functions are only overriden once.

Bug: 910284
Change-Id: I1bf203b45865275ec2cbf5e52ab388a5064de16e
Reviewed-on: https://chromium-review.googlesource.com/c/1356014Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: John Wu <jzw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612708}