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/1707293002

Cr-Original-Commit-Position: refs/heads/master@{#376288}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 62b5ce373ce17228a3c06f3e32ff329180e89921
1 file changed
tree: 408acd991bf85dfb9791bdd68fe0cc5b990361e5
  1. tools/