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There were two issues:
. Need to register the VariationsIdsProvider with the trial-registry.
  Without this the header content is never updated.
. Need to return true from VariationsClient::IsSignedIn(). This is
  needed as registerExternalExperimentIDs() supplies SignedIn.

I clearly need a test for this. Will add it in a follow up.

BUG=11478276
TEST=none

Change-Id: I62b6976dd51c3eb3aff4412ee2512b7c5516301b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532516Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826834}