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/1708513002 Cr-Original-Commit-Position: refs/heads/master@{#375970} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: 89d33db3bfd53fbb3d85bb218ad4db8489a4dc34