gn: Include source files outside the source root for Xcode workspace

This change unifies Xcode workspaces with projects for other IDEs.

BUG=n/a

Review-Url: https://codereview.chromium.org/2489673004
Cr-Original-Commit-Position: refs/heads/master@{#431856}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: b9f57fffd94bdfe264ddaf220dfecac603075540
2 files changed
tree: 470ec2511d030988b0c65fc87268472b8e340caa
  1. tools/