Make ParsedRequestCookie comprise strings.

For safety's sake, make a string copy rather than try to be
very sure about whether the string backing a StringPiece
will outlive the StringPiece.

BUG=582001

Change-Id: I92b1d7e541173f0622c3791cf0e1b751d1018b06
Reviewed-on: https://chromium-review.googlesource.com/c/1383216
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617888}
4 files changed