Fix a crash in HttpVaryData::MatchesRequests.
Fix a crash in HttpVaryData::MatchesRequests.

crbug.com/469675 remove an implicit Vary: Cookie for redirected
requests. But some cached redirected requests could still have
HttpVaryData::is_valid_ == true.

Therefore some requests could reach a NOTREACHED() in
HttpVaryData::MatchesRequests() because comparing an HTTP request
vary that have a HttpVaryData::Init() now returning false because
no Vary from the request with a previously cached HttpVaryRequest
that have is_valid_ == true.

BUG=611575

Review-Url: https://codereview.chromium.org/2000123002
Cr-Commit-Position: refs/heads/master@{#395317}
(cherry picked from commit 7ec544a858f4dd2b5b3a0ad0949a1f24f55f10b4)

Review URL: https://codereview.chromium.org/2012463002 .

Cr-Commit-Position: 03d981dee550804c6934fd1a1d9cfa3c172affd3@{#30}
Cr-Branched-From: e9621a186760eb36d74098d9e33e4dd91c427b34@{#394939}

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