gn2bp: add code that collects common sources

Once sources will be added to a Target.Arch, common sources need to be
collected from the arch-dependent object. This is done via finalize()
method, which is called once on every Target. Finalize collects the
intersection of all sources and subtracts it from the arch dependent
sources.

Test: //components/cronet/android:cronet
Change-Id: If2d0d04ff6c8df0efb541a322bcf2febcf79bb55
1 file changed