Remove linked_ptr from components/policy/*

Use std::unique_ptr instead of deprecated linked_ptr

Bug: 556939
Change-Id: Iefb3dcb3ae891fe4b0fd0dd70be408517bcd9d85
Reviewed-on: https://chromium-review.googlesource.com/1034942
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Commit-Queue: Sungguk Lim <limasdf@gmail.com>
Cr-Commit-Position: refs/heads/master@{#555303}
2 files changed