GN: Use std::unique_ptr for owning pointers in InputFileManager.

BUG=602726

Review URL: https://codereview.chromium.org/1877253003

Cr-Original-Commit-Position: refs/heads/master@{#388022}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 959ec4e4802a64b022d7101eafbc791a0339e971