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Adds Fieldtrial params to be used for fine-tuning the frecency score of
local repeatable queries. These params allow fine tuning of the age
threshold, recency decay, and frequency exponent of the score.

The default values match those in http://shortn/_VLAOJTxx6J

Bug: 1110077
Change-Id: I6d2cd1452d02c1133c7e2f6232b25168d81408da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2422427
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815437}