gn2bp: allow targets to depend on chromium's libc++ and libunwind

We need to determine if this should be replaced by Android's version of
those libraries.

This can be done via "stl" property on dependent targets (we would
additionally have to configure the version).

Test: n/a
Change-Id: Ibd54cc0b58086ec77d5eee40708f28b237d70c3f
1 file changed