GN: Use std::unique_ptr for owning pointers in Scope::NamedScopeMap.

BUG=602726

Review URL: https://codereview.chromium.org/1885513003

Cr-Original-Commit-Position: refs/heads/master@{#388002}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: ac5bf2c0d550e62f6cc58862990e7c4a1017cdf7