Add an `auto_profile_path` flag to GN to control AFDO.

build/config/compiler/compiler.gni

 # Copyright 2015 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/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 16 matching lines @@
   # with some utilities such as icecc and ccache. Requires gold and
   # gcc >= 4.8 or clang.
   # http://gcc.gnu.org/wiki/DebugFission
   #
   # This is a placeholder value indicating that the code below should set
   # the default.  This is necessary to delay the evaluation of the default
   # value expression until after its input values such as use_gold have
   # been set, e.g. by a toolchain_args() block.
   use_debug_fission = "default"
 
+  # AFDO (Automatic Feedback Directed Optimizer) is a form of profile-guided
+  # optimization that GCC supports. It used by ChromeOS in their official
+  # builds. To use it, set auto_profile_path to the path to a file containing
+  # the needed gcov profiling data.
+  auto_profile_path = ""

[brettw, 2016/09/27 23:52:41]

This appears in a different declare_args block in the previous file. That seems
wrong and I would have expected GN to give an error. Maybe we just want this
one?

[Dirk Pranke, 2016/09/28 00:08:34]

Oh, whoops, I forgot to back this change out. I did this change and then the
BUILD.gn one, at which point I realized we didn't need to expose the arg in the
.gni.

+
   # Tell VS to create a PDB that references information in .obj files rather
   # than copying it all. This should improve linker performance. mspdbcmf.exe
   # can be used to convert a fastlink pdb to a normal one.
   is_win_fastlink = false
 
   # Specify the current PGO phase, only used for the Windows MSVS build. 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.
@@ skipping 48 matching lines @@
     # Mac and Windows have them separate, so in Release Linux, default them off,
     # but keep them on for Official builds and Chromecast builds.
     symbol_level = 2
   } else if (using_sanitizer) {
     # Sanitizers require symbols for filename suppressions to work.
     symbol_level = 1
   } else {
     symbol_level = 0
   }
 }