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WebLayer will be using VariationsService but it's distinct from android
chrome so we need a way to override the platform in the variations url
request.

Bug: 1025612
Change-Id: Ie4f9f6bfabc3a6b9234fcf5589ad9f5b98c3224f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1951012
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736953}