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This CL implements Fetchers which can call endpoints defined for the
IDP IFrame protocol, specifically listSessions and getToken. This will
be used as part of the Cross-Device Promo so that Chrome can mint
access tokens of a particular scope based on cookies.

BUG=463611

Review URL: https://codereview.chromium.org/973953002

Cr-Commit-Position: refs/heads/master@{#319829}