tools/gn: replace usage of linked_ptr by using scoped_ptr
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: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#362520}

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