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Based on the foundation from https://crrev.com/c/1967245 this patch
adds support for variations headers to WebLayer along with a simple
browser test.

Bug: 1025612
Change-Id: Ic1dda5ccd2581c15a5d51094ece277e3035a866e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2041663
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745424}