[Clean up] Move VariationsHttpHeaderProvider into a subdirectory.

components/metrics/BUILD.gn

Index: components/metrics/BUILD.gn
diff --git a/components/metrics/BUILD.gn b/components/metrics/BUILD.gn
index 8bccd4582ab779b9f312198c973cd66462e51bba..13c34a941939b5358c849c55b9644488cde42c39 100644
--- a/components/metrics/BUILD.gn
+++ b/components/metrics/BUILD.gn
@@ -138,4 +138,27 @@ if (is_linux) {
   }
 }
 
+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