[Android] Include NestedSystemMessageHandler in gn builds of content_shell_apk.

android_webview/test/BUILD.gn

 # 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")
 import("//testing/test.gni")
 
 # Mark all targets as test only.
 testonly = true
 
@@ skipping 236 matching lines @@
 }
 
 android_assets("android_webview_unittests_assets") {
   sources = [
     "unittest/assets/asset_file.ogg",
   ]
 }
 
 # GYP: //android_webview/android_webview_tests.gypi:android_webview_unittests_java
 android_library("android_webview_unittests_java") {
+  testonly = true
   deps = [
     "//android_webview:android_webview_java",
     "//base:base_java",
     "//base:base_java_test_support",
     "//content/public/test/android:content_java_test_support",
   ]
   java_files = [
     "../unittestjava/src/org/chromium/android_webview/unittest/InputStreamUnittest.java",
     "../unittestjava/src/org/chromium/android_webview/unittest/MockAwContentsClientBridge.java",
   ]
 }
 
 # GYP: //android_webview/android_webview_tests.gypi:android_webview_unittests_jni
 generate_jni("android_webview_unittests_jni") {
   jni_package = "android_webview_unittests"
   sources = [
     "../unittestjava/src/org/chromium/android_webview/unittest/InputStreamUnittest.java",
     "../unittestjava/src/org/chromium/android_webview/unittest/MockAwContentsClientBridge.java",
   ]
 }
 
 # GYP: //android_webview/android_webview_tests.gypi:libdrawgl
 shared_library("libdrawgl") {
   sources = [
     "shell/src/draw_gl/draw_gl.cc",
   ]
   configs -= [ "//build/config/android:hide_native_jni_exports" ]
 }