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