Sync remaining components_unittests with GN

components/language_usage_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.
 
 source_set("language_usage_metrics") {
   sources = [
     "language_usage_metrics.cc",
     "language_usage_metrics.h",
   ]
 
   deps = [
     "//base",
   ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "language_usage_metrics_unittest.cc",
+  ]
+
+  deps = [
+    ":language_usage_metrics",
+    "//testing/gtest",
+  ]
+}