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The status quo implementation generates two types,
cached_variation_ids_header_signed_in_ and cached_variation_ids_header_,
which are stored in strings.

In order to limit the number of VariationIDs included in the header in
third-party contexts, four headers are needed. The headers are stored in
a map.

FYI: While GetClientDataHeader()'s |web_visibility| argument is only
Study_GoogleWebVisibility_ANY for the time being, this will not always
be the case.

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: Ifdc502fbc25fc3b09911414198022e059132a83c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2388763Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Caitlin Fischer <caitlinfischer@google.com>
Cr-Commit-Position: refs/heads/master@{#807875}