absolute path format inconsistency

GN's absolute path always uses `/` even on [Windows](https://source.chromium.org/gn/gn/+/main:src/gn/source_dir.h;l=26-27).
This issue was suppressed before because GN static library was never built in debug mode and this [DCHECK](https://source.chromium.org/gn/gn/+/main:src/gn/source_file.cc;l=14-22) is never reached.