Add the possibility to build with PGO when using Clang

build/config/BUILD.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.
 
 import("//build/config/allocator.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/dcheck_always_on.gni")
 import("//build/config/features.gni")
+import("//build/config/pgo/pgo.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
 import("//build/toolchain/goma.gni")
 
 # One common error that happens is that GYP-generated headers within gen/ get
 # included rather than the GN-generated ones within gen/ subdirectories.
 # TODO(GYP_GONE): Remove once GYP is gone (as well as exec_script exception).
 assert(
     exec_script("//build/dir_exists.py", [ "obj.host" ], "string") == "False",
     "GYP artifacts detected in $root_build_dir.$0x0A" +
@@ skipping 397 matching lines @@
       # file doesn't exist, no error will be generated (probably MS tested this
       # case but forgot the other one?). To reproduce this error, do a build,
       # then delete the precompile.c.obj file, then build again.
       cflags_c = [ "/wd4206" ]
     } else if (is_mac) {
       precompiled_header = "build/precompile.h"
       precompiled_source = "//build/precompile.h"
     }
   }
 }