Make crazy linker work in gn build

mojo/android/BUILD.gn

 # Copyright 2014 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")
 
 group("android") {
   testonly = true
   deps = [
     ":system_java",
     ":mojo_javatests",
   ]
 }
 
 generate_jni("system_java_jni_headers") {
   sources = [
     "system/src/org/chromium/mojo/system/impl/CoreImpl.java",
   ]
 
   jni_package = "mojo"
 }
 
-shared_library("libsystem_java") {
+source_set("libsystem_java") {
   sources = [
     "system/core_impl.cc",
     "system/core_impl.h",
   ]
 
   deps = [
     ":system_java_jni_headers",
     "//mojo/edk/system",
     "//mojo/environment:chromium",
   ]
@@ skipping 42 matching lines @@
 
   deps = [
     ":system_java",
     "//base:base_java",
     "//base:base_java_test_support",
     "//mojo/public/interfaces/bindings/tests:test_interfaces_java",
     "//mojo/public/java:bindings",
     "//mojo/public/java:system",
   ]
 }