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This CL detects and allows signed exchange cache URLs even if they are
lookalikes.

Signed Exchange cache URLs (the initial URL that redirects to the
SXG) serve application/signed-exchange Content-Type response header.
In this case, the navigation will always either fail or redirect to
the primary URL (getting the content either from inside the SXG or the
network depending on whether the validations succeed).

Bug: 1110151
Change-Id: I5a15c0248402e563bb34b9643c483d80770bd121
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2441276
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Reviewed-by: Joe DeBlasio <jdeblasio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818052}