regenerating build files is currently broken with a tip-of-tree GN (one containing r392491).

do this:

% gn gen out/foo
% ninja -C out/foo gn
% mkdir out/bar
% echo "is_debug=false" > out/bar/args.gn
% out/foo/gn gen out/bar
% touch out/bar/args.gn
% ninja -C out/bar -d explain build.ninja
ninja: Entering directory `out/bar'
ninja explain: output build.ninja older than most recent input /media/src/src/c/src/out/bar/args.gn (1463108607 vs 1463108615)
[1/1] Regenerating ninja files
ninja explain: output build.ninja older than most recent input /media/src/src/c/src/out/bar/args.gn (1463108607 vs 1463108615)
[1/1] Regenerating ninja files
ninja explain: output build.ninja older than most recent input /media/src/src/c/src/out/bar/args.gn (1463108607 vs 1463108615)
[1/1] Regenerating ninja files
^C
%

(or some such ... the ninja regeneration loops indefinitely until it gives up).

The problem is in 

https://code.google.com/p/chromium/codesearch?q=ninja_build_writer.cc#chromium/src/tools/gn/ninja_build_writer.cc&l=176

By changing from unconditionally writing build.ninja to WriteFileIfChanged(), we don't update build.ninja when one of the dependencies is out of date. this means that build.ninja will always be out of date (args.gn doesn't actually matter in the example above, it could be any build file).

This is the reason the roll failed in 

https://codereview.chromium.org/1966613002/

Since nothing is using ToT GN yet, I'll leave the faulty CL in for now and leave the fix as an exercise to the reader, but we should fix this tomorrow so we're not blocked and can get the other fixes rolled.