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This patch implements the following restrictions on Signed Exchange
inner responses, added in https://github.com/WICG/webpackage/pull/354:

- Inner response must have a Content-Type header (step 7. of [1]).
- Clients must not sniff a media type from the payload bytes.
  In the loading spec, parser injects "X-Content-Type-Options: nosniff"
  header while parsing response headers (step 8. of [1]) to achieve
  this.

[1] https://wicg.github.io/webpackage/loading.html#parse-b2-cbor-headers

Bug: 1051390
Change-Id: Ia7274f32e43dac28affd718a2965d575b955353c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2059751Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742451}