Add gn_to_bp.py.

The general idea here is, run GN in --ide=json mode to get most information.

Then, read a couple .gni files to get the rest (platform specific source lists, Android framework defines).

For now, I'm generating Android.bp and SkUserConfig.h.  I figure we can do DM and nanobench once these work?

Change-Id: I8e7f60d6572f2d4769760cf872895518a15d841b
Reviewed-on: https://skia-review.googlesource.com/5554
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 790dd21..29acd54d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -697,6 +697,11 @@
 
 # Targets guarded by skia_enable_tools may use //third_party freely.
 if (skia_enable_tools) {
+  # Used by gn_to_bp.py to list our public include dirs.
+  source_set("public") {
+    configs += [ ":skia_public" ]
+  }
+
   config("skia.h_config") {
     include_dirs = [ "$target_gen_dir" ]
   }