tools/gn: replace usage of linked_ptr by using scoped_ptr

With C++11, we can use scoped_ptr in STL containers now.

BUG=556939
TEST=gn gen + gn_unittests
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/1480423003

Cr-Commit-Position: refs/heads/master@{#362520}
1 file changed