Support for source_target_relative expansion in GN

This can be used to get a path to the source file relative to the target's
directory.

Review-Url: https://codereview.chromium.org/2387763002
Cr-Original-Commit-Position: refs/heads/master@{#439541}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7d88638248b10cec98364cf561f268cbed62587e
12 files changed
tree: 0c7c5189405856866dd52fbb80158a278e223aee
  1. tools/