Writing files from gyp files in Makefile mode uses the undocumented $(obj) variable

[GoogleCodeExporter]

If you write a file with the <| operator and the written data contain data from 
SHARED_INTERMEDIATE_DIR, you get $(obj)/gen in the output with no specification 
what $(obj) might be.

In ninja mode you get PRODUCT_DIR instead which is a predefined variable in gyp 
and can be inspected.

I think $(obj) and PRODUCT_DIR are the same but obj is not documented.

I suspect the format is aimed to suit ninja vs gyp but in bug 424192 I need to 
parse those output files myself.

Originally reported as 
https://code.google.com/p/chromium/issues/detail?id=472602 in the chromium 
issue tracker.

Original issue reported on code.google.com by brat...@opera.com on 14 Apr 2015 at 1:14