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Starting implementation of go/auto-upgrade-mixed, adds the variations
flag and auto upgrades all mixed content (with no fallback) if it is
set. Further CLs will filter by mixed content type (blockable vs
optionally blockable) depending on variations parameters, and will
add fallback.

Change-Id: Ic14124fe21ba08dc34738fdedebe2a37fb73ac3d
Reviewed-on: https://chromium-review.googlesource.com/1249094
Commit-Queue: Carlos IL <carlosil@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595691}