Make GN --as=output relative to the root build directory.

The intended use for this parameter is to give files that can be passed to Ninja to compile. Ninja takes files relative to the root build directory.

BUG=546127

Review URL: https://codereview.chromium.org/1413573006

Cr-Original-Commit-Position: refs/heads/master@{#355555}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 1c010e5f6ccb6b5d65044500fb1f656d3f8b5d6b
2 files changed
tree: 73a53e705cf5696605008bf96344cf8d9095f172
  1. tools/