Add gyp and gn targets for jFormatString.jar needed by errorprone.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..eb1b2976a7cad3f5b7c5ef885796d4a1b6d433f3
--- /dev/null
+++ b/BUILD.gn
@@ -0,0 +1,10 @@
+# Copyright 2015 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/config/android/rules.gni")
+
+# GYP: //third_party/findbugs:format_string_jar
+java_prebuilt("format_string_java") {
+  jar_path = "lib/jFormatString.jar"
+}