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This CL enables Mirror for the extension consent flow:
- Makes the account settings dialog appear during the consent flow when
  Gaia passes a Mirror response header. This allows the user to modify
  the list of accounts on their device.
- Keeps the consent flow window opened when it happens. In the case
  the user don't want to modify their accounts, they could go back to
  the consent flow.

Bug: 1026237
Change-Id: I3cdd8bb868e7ec8b80f9cd59f45aee2d06616b1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2144200Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758789}