This was only necessary when the Engine had to build in multiple buildroots
where the sources where checked out at different paths relative to the
buildroot. This is no longer the case and there are already cases GN rules
have been written that mix and match variable usage with the direct
specification of the path to the Flutter sources relative to the sole buildroot.
Flow is a simple compositor based on Skia that the Flutter engine uses to cache
recoded paint commands and pixels generated from those recordings. Flow runs on
the GPU thread and uploads information to the GPU.