Add lots of testonly annotations to the GN build.

content/test/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/features.gni")
 import("//build/config/ui.gni")
 
 content_tests_gypi_values = exec_script(
     "//build/gypi_to_gn.py",
     [ rebase_path("../content_tests.gypi"),
       "--replace=<(SHARED_INTERMEDIATE_DIR)=$root_gen_dir" ],
     "scope",
     [ "../content_tests.gypi" ])
 
 # GYP version //content/content_tests.gypi:test_support_content
 static_library("test_support") {
+  testonly = true
   deps = [
     "//content/public/app",
     "//content/public/browser",
     "//content/public/common",
     "//net:test_support",
     "//skia",
     "//storage/common",
     "//testing/gmock",
     "//testing/gtest",
     "//ui/accessibility:ax_gen",
@@ skipping 105 matching lines @@
       "test/test_content_browser_client.cc",
       "test/test_content_client.cc",
     ]
   }
 }
 
 if (!is_ios) {
 
   # GYP version //content/content_tests.gypi:layouttest_support_content
   static_library("layouttest_support") {
+    testonly = true
     sources = rebase_path(content_tests_gypi_values.layouttest_support_content_sources,
                           ".", "//content")
 
     deps = [
       ":test_support",
       "//skia",
       "//v8",
       "//ui/accessibility:ax_gen",
     ]
 
     if (is_android) {
       #deps += [ "test_support_content_jni_headers" ]  TODO(GYP)
     }
   }
 
 }  # !is_ios
 
 # TODO(GYP): Unit test targets