Restrict `AssociateGoogleVariationID` using `base::PassKey`.

The `AssociateGoogleVariationID` functions are now restricted to be called only from `VariationsSeedProcessor` and `SyntheticTrialRegistry` using `base::PassKey` for privacy reasons. Tests now use a new `AssociateGoogleVariationIDForTesting` function.

Additionally, some clean ups on the APIs are performed, such as
making the other params from the two passkey callsites consistent.

Note: This CL is not sufficient to fully restrict things as there are
additional APIs that need to be tightened up, that will be done in
future CLs.

Bug: 315411418
Change-Id: Iec8025e3a1b36abcb38bb823da32a65418faf56c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6941003
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Olivia Saul <osaul@google.com>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1515004}