Before this patch, Content-Encoding header of inner response was not
checked (content was assumed to be "mi-sha256-03" encoded).

Currently we only support "mi-sha256-03" inner encoding, so this patch
makes SignedExchangeHandler reject (causing fallback redirect) the
signed exchange if its inner Content-Encoding is not just "mi-sha256-03".

This also refactors the logic to check integrity scheme, by moving them
into single place, SignedExchangeHandler::CreateResponseBodyStream().

Note: This shouldn't affect amppackager, because it strips additional
content-encodings in b3.
https://github.com/ampproject/amppackager/issues/243

Bug: 931055, 934629
Change-Id: Icc0bde36194d4a387ab30968cccf1bef4c81fe60
Reviewed-on: https://chromium-review.googlesource.com/c/1482351Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635413}