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Adds a default-disabled Feature to override the policy that Contextual
Search applies to plain HTTP pages. This allows disabling the recent
change that treats HTTP just like HTTPS landed in
https://crrev.com/c/2393526. Enabling the Feature will allow us to go
back to the legacy policy, and allow us to measure the impact of the
policy change accurately.

BUG=1124397,1129969

Change-Id: If9ceab183053a573b45a3db6051b55c625c21755
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2422264
Commit-Queue: Donn Denman <donnd@chromium.org>
Commit-Queue: Theresa  <twellington@chromium.org>
Auto-Submit: Donn Denman <donnd@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809473}