[Loader] Add asserts about revalidation and redirects
[Loader] Add asserts about revalidation and redirects

After https://codereview.chromium.org/2011283002/ and
https://codereview.chromium.org/2017883002/, redirect chain must be empty
when revalidation starts or during revalidation is ongoing.
This CL adds asserts to verify that.

BUG=613971, 614989

Review-Url: https://codereview.chromium.org/2020253002
Cr-Commit-Position: refs/heads/master@{#397683}
(cherry picked from commit d0753f72046d160bc34584a47eb3f3a3c56daff1)

Review URL: https://codereview.chromium.org/2046923002 .

Cr-Commit-Position: 03d981dee550804c6934fd1a1d9cfa3c172affd3@{#275}
Cr-Branched-From: e9621a186760eb36d74098d9e33e4dd91c427b34@{#394939}

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