GN: Use std::unique_ptr for owned pointers in ImportManager. BUG=602726 Review URL: https://codereview.chromium.org/1886453002 Cr-Original-Commit-Position: refs/heads/master@{#387997} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: 647a1af223628bb4e15b69a29ca257477282b737