Add base::FeatureParam<> struct

base/BUILD.gn

 # Copyright (c) 2013 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.
 
 # HOW TO WRITE CONDITIONALS IN THIS FILE
 # ======================================
 #
 # In many other places, one would write a conditional that expresses all the
 # cases when a source file is used or unused, and then either add or subtract
 # it from the sources list in that case
@@ skipping 2438 matching lines @@
 
 if (enable_nocompile_tests) {
   nocompile_test("base_nocompile_tests") {
     sources = [
       "bind_unittest.nc",
       "callback_list_unittest.nc",
       "callback_unittest.nc",
       "containers/span_unittest.nc",
       "memory/ref_counted_unittest.nc",
       "memory/weak_ptr_unittest.nc",
+      "metrics/field_trial_params_unittest.nc",
       "metrics/histogram_unittest.nc",
       "strings/string16_unittest.nc",
       "task_scheduler/task_traits_unittest.nc",
     ]
 
     deps = [
       ":base",
       "//base/test:run_all_unittests",
       "//testing/gtest",
     ]
@@ skipping 334 matching lines @@
 }
 
 fuzzer_test("base_json_correctness_fuzzer") {
   sources = [
     "json/correctness_fuzzer.cc",
   ]
   deps = [
     ":base",
   ]
 }