[Clean up] Move VariationsHttpHeaderProvider into a subdirectory.

components/metrics/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.
 
 # GYP version: components/metrics.gypi:metrics
 source_set("metrics") {
   sources = [
     "clean_exit_beacon.cc",
     "clean_exit_beacon.h",
     "client_info.cc",
@@ skipping 120 matching lines @@
     sources = [
       "serialization/serialization_utils.cc",
       "serialization/serialization_utils.h",
       "serialization/metric_sample.cc",
       "serialization/metric_sample.h",
     ]
     deps = [ "//base" ]
   }
 }
 
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "compression_utils_unittest.cc",
+    "daily_event_unittest.cc",
+    "machine_id_provider_win_unittest.cc",
+    "metrics_hashes_unittest.cc",
+    "metrics_log_manager_unittest.cc",
+    "metrics_log_unittest.cc",
+    "metrics_reporting_scheduler_unittest.cc",
+    "metrics_service_unittest.cc",
+    "metrics_state_manager_unittest.cc",
+    "persisted_logs_unittest.cc",
+    "profiler/profiler_metrics_provider_unittest.cc",
+  ]
+
+  deps = [
+    ":metrics",
+    "//base/test:test_support",
+    "//testing/gtest",
+  ]
+}
+
 # TODO(GYP): metrics_chromeos