Add lots of testonly annotations to the GN build.

ppapi/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.
 
 gypi_values = exec_script(
     "//build/gypi_to_gn.py",
     [ rebase_path("ppapi_sources.gypi") ],
     "scope",
     [ "ppapi_sources.gypi" ])
 
@@ skipping 762 matching lines @@
     ":ppapi_shared",
     "//base",
     "//ipc",
     "//media:shared_memory_support",
     "//ui/surface",
     "//url",
   ]
 }
 
 static_library("ppapi_unittest_shared") {
+  testonly = true
   sources = [
     "proxy/ppapi_proxy_test.cc",
     "proxy/ppapi_proxy_test.h",
     "proxy/resource_message_test_sink.cc",
     "proxy/resource_message_test_sink.h",
     "shared_impl/test_globals.cc",
     "shared_impl/test_globals.h",
     "shared_impl/unittest_utils.cc",
     "shared_impl/unittest_utils.h",
   ]
 
   deps = [
     ":ppapi_proxy",
     ":ppapi_shared",
     "//base/test:test_support",
     "//ipc",
     "//ipc:test_support",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }
 
 # TODO(GYP) other targets from ppapi_tests.gyp