Add the possibility to build with PGO when using Clang

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 942605e38e33022ef00ce2b6066ef71e974dd411..9c37ddadf9b78046f6c389718356f4c999ace6f1 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -478,6 +478,7 @@ default_compiler_configs = [
   "//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) {