gn2bp: change filegroup to cc_defaults

Filegroups do not support per-target/arch sources, but cc_defaults do.
Since modules that are not compiled (filegroups, genrules, cc_defaults)
do not support deps, only cc_library or cc_binary targets can depend on
filegroups. This means, it should be safe to convert filegroups to
cc_defaults.

Test: //components/cronet/android:cronet
Change-Id: Ie937102c093d46d42867e0472e43901fdf427894
2 files changed