GN: Use std::unique_ptr for owned pointers in VisualStudioWriter. BUG=602726 Review URL: https://codereview.chromium.org/1883593004 Cr-Original-Commit-Position: refs/heads/master@{#388031} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: 31500019dff1f12beb9731da14db3820cba2b991