GN: Replace vector<ParseNode*> with vector<unique_ptr<ParseNode>> in parse_tree.h. Chromium can now use std::unique_ptr. BUG=602726 Review URL: https://codereview.chromium.org/1884503003 Cr-Original-Commit-Position: refs/heads/master@{#387958} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: 6ce99c7025ebaca2747f659da3700a388f627919