Add the possibility to build with PGO when using Clang

build/config/compiler/pgo/pgo.gni

+# Copyright 2016 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() {
+  # Specify the current PGO phase.
+  # Here's the different values that can be used:
+  #     0 : Means that PGO is turned off.
+  #     1 : Used during the PGI (instrumentation) phase.
+  #     2 : Used during the PGO (optimization) phase.
+  #
+  # TODO(sebmarchand): Add support for the PGU (update) phase.
+  chrome_pgo_phase = 0
+
+  # When using chrome_pgo_phase = 2, read profile data from this path.
+  pgo_data_path = ""
+}