Minor fixes to junit and robolectric BUILD.gn files.

testing/android/junit/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.
 
 assert(is_android)
 
 import("//build/config/android/rules.gni")
 
 # GYP: //testing/android/junit_test.gyp:junit_test_support
 java_library("junit_test_support") {
+  testonly = true
   DEPRECATED_java_in_dir = "java/src"
   deps = [
     "//third_party/junit",
     "//third_party/mockito:mockito_java",
     "//third_party/robolectric:robolectric_java",
   ]
 }
 
 # GYP: //testing/android/junit_test.gyp:junit_unit_tests
 java_binary("junit_unittests") {
+  testonly = true
   deps = [
     ":junit_test_support",
     "//third_party/junit",
   ]
   main_class = "org.chromium.testing.local.JunitTestMain"
   DEPRECATED_java_in_dir = "javatests/src"
 }