gn2bp: collect linker unit deps per target

Both transitive static libraries and LINKER_UNIT_TYPES dependencies need
to be stored separately for every target as they could differ. In
reality, only static deps differ right now.

I am not a big fan of how transitive_static_libs_deps are handled in
this CL; maybe we can come up with a more readable solution.

Test: //components/cronet/android:cronet
Change-Id: Ia4ad8e264b1243b7dd5d58a26543ee78206f9c79
2 files changed