GN: Use std::unique_ptr in Loader::ToolchainRecordMap.

BUG=602726

Review URL: https://codereview.chromium.org/1878353002

Cr-Original-Commit-Position: refs/heads/master@{#388015}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: f9b6b5ae0cd8396663e5c1b06468384fc9d89688