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The feature is disabled by default.

Also, rename two existing IDCollectionKeys.

This change paves the way for limiting the set of experiment IDs
included in X-Client-Data headers sent in third-party contexts.

Bug: 1094303
Change-Id: Ib535b0b8657aeb82570c552b31d722bd30168019
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363467
Commit-Queue: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800310}