gn yasm: use root_build_dir instead of . for rebase_path

I think these should be relative to the root_build_dir not the
dir (which is what "." is right?). It was working before if you
happen to use (say) out\Default as the GN directory, but I've been using
out_gn so there was an extra .. in the path.

Also, a missing size_t->int warning disable on Windows.

Review URL:

Cr-Original-Commit-Position: refs/heads/master@{#289813}
Cr-Mirrored-Commit: 25a24e96549586a99aec858da91db608fd28bf1d
1 file changed