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
1 file changed
tree: a54105611a658252871f501577e03171f250b88f
  1. tools/