Do not set 'ARCHS' key, it's not implemented by ninja generator.

Normally it should be ignored when using ninja generator and does
nothing, but when I was building atom-shell on my OS X 10.8.5 machine,
it was causing bad .ninja files to be generated.

Will investigate more on this when we are going to use latest brightray.

Conflicts:
	brightray.gypi
Use atom branch of libchromiumcontent.