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This CL implements the time decay for tile score. The equation is
score = original_score * exp(-0.099 * time_passed).

BUG=1096224

Change-Id: Id623232a7510f2ca63f854e75846bc6119fdba03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2417497
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808632}