Add a flag to turn on incremental LTCG.

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 30 matching lines @@
   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.
   #
   # TODO(sebmarchand): Add support for the PGU (update) phase.
   chrome_pgo_phase = 0
+
+  # Whether or not we should turn on incremental LTCG. Only affects the VS
+  # Windows build.
+  use_incremental_ltcg = false

[brucedawson, 2016/11/03 18:13:19]

One could argue that this should be named "use_incremental_wpo" for consistency
with the full_wpo_on_official flag. But, I can see a case for either name, so it
is your call.

[Sébastien Marchand, 2016/11/03 21:18:02]

tough call, I'll go with the wpo variant as these 2 flags will almost always be
used together.

 }
 
 declare_args() {
   # Whether or not the official builds should be built with full WPO. Enabled by
   # default for the PGO and the x64 builds.
   if (chrome_pgo_phase > 0) {
     full_wpo_on_official = true
   } else {
     full_wpo_on_official = false
   }
@@ skipping 35 matching lines @@
     symbol_level = 0
   }
 }
 
 # Assert that the configuration isn't going to hit https://crbug.com/648948.
 assert(ignore_elf32_limitations || !is_android || android_64bit_target_cpu ||
            is_component_build || symbol_level < 2 || is_clang,
        "Android 32-bit non-component, non-clang builds cannot have " +
            "symbol_level=2 due to 4GiB file size limit, see " +
            "https://crbug.com/648948")