Add support for robolectric play-services-shadows.

chrome/android/webapk/test/BUILD.gn

 # Copyright 2016 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")
 
 java_library("junit_test_support") {
+  # Skip platform checks since Robolectric depends on requires_android targets.
+  bypass_platform_checks = true
   testonly = true
   java_files = [ "src/org/chromium/webapk/test/WebApkTestHelper.java" ]
   deps = [
     "//third_party/robolectric:robolectric_all_java",
   ]
 }