Add the possibility to build with PGO when using Clang

build/config/BUILDCONFIG.gn

 # Copyright (c) 2013 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.
 
 # =============================================================================
 # WHAT IS THIS FILE?
 # =============================================================================
 #
 # This is the master GN build configuration. This file is loaded after the
 # build args (args.gn) for the build directory and after the toplevel ".gn"
@@ skipping 460 matching lines @@
   "//build/config/compiler:compiler",
   "//build/config/compiler:clang_stackrealign",
   "//build/config/compiler:compiler_arm_fpu",
   "//build/config/compiler:chromium_code",
   "//build/config/compiler:default_include_dirs",
   "//build/config/compiler:default_optimization",
   "//build/config/compiler:default_stack_frames",
   "//build/config/compiler:default_symbols",
   "//build/config/compiler:no_rtti",
   "//build/config/compiler:runtime_library",
+  "//build/config/pgo:default_pgo_flags",

[Sébastien Marchand, 2016/11/17 17:31:15]

My concern with this is that it seems to be a global way to enable PGO on *all*
the targets? We really want to be able to turn on PGO only for some targets (on
Windows at least), so I'm all in for moving the PGO specific code into a build
config (instead of having it living in chrome/BUILD.gn), but I don't think that
it should be a part of the default config? 

   "//build/config/sanitizers:default_sanitizer_flags",
 ]
 if (is_win) {
   default_compiler_configs += [
     "//build/config/win:default_crt",
     "//build/config/win:lean_and_mean",
     "//build/config/win:nominmax",
     "//build/config/win:unicode",
     "//build/config/win:winver",
     "//build/config/win:vs_code_analysis",
@@ skipping 151 matching lines @@
 set_defaults("component") {
   if (is_component_build) {
     configs = default_shared_library_configs
     if (is_android) {
       configs -= [ "//build/config/android:hide_native_jni_exports" ]
     }
   } else {
     configs = default_compiler_configs
   }
 }