chromium: Fix build of stable channel.

Build failure on Hydra:

https://hydra.nixos.org/build/9823160

This was caused by the update of file in 5885709.

As file seems to be used for only one substition in the gyp files, we
can now drop the build dependency on file and patch out the substition
expression, as it is done before actually testing if the value has been
set by -D (gyp, y u no have lazy eval!?).

PS: Proudly untested against beta and dev channels, redeployed my own
Hydra and building on my workstation here really is ... annoying (lavg
41 on a system with nproc 8, less than 8 GB RAM and you probably will
have as much "fun" as I just had writing this commit mess...a....g

FUCK^H^H^H^H^H^H...e).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>