Initial attempt at a GN/Ninja-based build system

BUG=20491586
diff --git a/BUILD.gn b/BUILD.gn
new file mode 100644
index 0000000..1af5ea4
--- /dev/null
+++ b/BUILD.gn
@@ -0,0 +1,6 @@
+# This pulls in main/BUILD.gn and all of its dependencies.
+group("bluetooth") {
+  deps = [
+    "//main:bluetooth.default",
+  ]
+}