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
1 file changed
tree: 4b69596c06b642ab180a373a8920774e3d169d09
  1. tools/