Port custom_tabs_client from GYP to GN

tools/android/android_tools.gyp

 # Copyright (c) 2012 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.
 
 {
   'targets': [
     # Intermediate target grouping the android tools needed to run native
     # unittests and instrumentation test apks.
     {
       # GN: //tools/android:android_tools
@@ skipping 32 matching lines @@
       ],
     },
     {
       'target_name': 'ps_ext',
       'type': 'none',
       'dependencies': [
         'ps_ext/ps_ext.gyp:ps_ext',
       ],
     },
     {
+      # GN: //tools/android:customtabs_benchmark
       'target_name': 'customtabs_benchmark',
       'type': 'none',
       'dependencies': [
         'customtabs_benchmark/customtabs_benchmark.gyp:customtabs_benchmark_apk',
       ],
     },
   ],
 }