gn-build: Add BUILD files for libchrome

This CL adds GN build files for libchrome, which is the ChromeOS/Brillo library
for many C++ tools, include libbase. I added a new GN build target for "base"
under secondary sources with a GN build file for libchrome which currently lives
in platform/external/libchrome in AOSP.

To build, clone libchrome into third_party/libchrome and remove
third_party/libchrome/base/BUILD.gn. Also clone external/modp_b64 into
third_party/modp_b64. Eventually I will have scripts set up to do
this automatically.

This means that we can start developing against libbase while running GN builds
on Goobuntu but this would break the tree until libchrome is buildable within
the Android build system (http://r.android.com/158392). So Brillo/Android builds
will depend on libchrome using the conventional Android.mk means.

Bug: 22175181
Change-Id: I9a737aa785488e5bc0ca1adc087d1d5dfcdf3bea