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This is achieved by talking to a different endpoint for fetching the
OGB sources, which accepts cookies. As a consequence, the cached data
is now cleared when the Gaia cookies change, rather than when the
SigninManager state changes.

Bug: 740462
Change-Id: Id230a118507e32e6f1a17c103c93c8eb5ccb0ba6
Reviewed-on: https://chromium-review.googlesource.com/595990
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Chris Pickel <sfiera@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492008}