GN: Normalize the build directory path BUG=586906 TEST=ln -s /tmp out_tmp && out/gn/gn gen //out_tmp/Release Review URL: https://codereview.chromium.org/1723913003 Cr-Original-Commit-Position: refs/heads/master@{#377175} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: f1c6101de0ecd583fd47fdc69a2a241367e49b7b