Add support for Blimp engine metrics reporting in development.

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.
 
+declare_args() {
+  # Overrides os name in uma metrics log to "Blimp".
+  metrics_use_blimp = false
+}
+
 # GYP version: components/metrics.gypi:metrics
 source_set("metrics") {
   sources = [
     "call_stack_profile_metrics_provider.cc",
     "call_stack_profile_metrics_provider.h",
     "clean_exit_beacon.cc",
     "clean_exit_beacon.h",
     "client_info.cc",
     "client_info.h",
     "cloned_install_detector.cc",
@@ skipping 325 matching lines @@
   ]
 
   if (is_linux) {
     sources += [ "serialization/serialization_utils_unittest.cc" ]
     deps += [ ":serialization" ]
   }
 
   if (is_chromeos) {
     deps += [ ":leak_detector_unit_tests" ]
   }
+
+  # These are only used by the blimp team, which has entirely migrated to gn,
+  # so this logic is not replicated in the gyp file.
+  if (metrics_use_blimp) {
+    defines = [
+      "OVERRIDE_OS_NAME_TO_BLIMP",
+      "ENABLE_REPORTING_BLIMP",
+    ]
+  }
 }
 # TODO(GYP): metrics_chromeos