Add support for single sample metrics.

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 561 matching lines @@
     "metrics/persistent_histogram_allocator.cc",
     "metrics/persistent_histogram_allocator.h",
     "metrics/persistent_memory_allocator.cc",
     "metrics/persistent_memory_allocator.h",
     "metrics/persistent_sample_map.cc",
     "metrics/persistent_sample_map.h",
     "metrics/sample_map.cc",
     "metrics/sample_map.h",
     "metrics/sample_vector.cc",
     "metrics/sample_vector.h",
+    "metrics/single_value_histograms.cc",
+    "metrics/single_value_histograms.h",
     "metrics/sparse_histogram.cc",
     "metrics/sparse_histogram.h",
     "metrics/statistics_recorder.cc",
     "metrics/statistics_recorder.h",
     "metrics/user_metrics.cc",
     "metrics/user_metrics.h",
     "metrics/user_metrics_action.h",
     "native_library.cc",
     "native_library.h",
     "native_library_ios.mm",
@@ skipping 2103 matching lines @@
 }
 
 fuzzer_test("base_json_correctness_fuzzer") {
   sources = [
     "json/correctness_fuzzer.cc",
   ]
   deps = [
     ":base",
   ]
 }