Introduce and use `AssociateGoogleVariationIDForTesting`.

This change introduces `AssociateGoogleVariationIDForTesting` as a test-only alias for associating Google Variation IDs. All test-related calls to `AssociateGoogleVariationID` are migrated to use this new function. This helps in distinguishing between test and production usages of this API.

Bug: 315411418
Change-Id: I364a8f9ecf84f6357a5765de0a8bf6a33779e6ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6932267
Reviewed-by: Colin Blundell <[email protected]>
Commit-Queue: Alexei Svitkine <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1514326}
NOKEYCHECK=True
GitOrigin-RevId: 3ee5d16153080f3858e7fb859155a63bad42e7b0
1 file changed