Add a flag to turn on incremental LTCG.

build/config/compiler/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/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 1228 matching lines @@
     ]
   }
   common_optimize_on_ldflags = []
   if (!is_component_build) {
     common_optimize_on_ldflags += [ "/OPT:ICF" ]  # Redundant COMDAT folding.
   }
   if (is_official_build) {
     common_optimize_on_ldflags += [ "/OPT:REF" ]  # Remove unreferenced data.
     if (!use_lld) {
       common_optimize_on_ldflags += [
-        "/LTCG",  # Link-time code generation.
-
         # Set the number of LTCG code-gen threads to eight. The default is four.
         # This gives a 5-10% link speedup.
         "/cgthreads:8",
       ]
+      if (use_incremental_ltcg) {
+        # Incremental Link-time code generation.
+        common_optimize_on_ldflags += [ "/LTCG:INCREMENTAL" ]
+      } else {
+        common_optimize_on_ldflags += [ "/LTCG" ]  # Link-time code generation.
+      }
     }
     if (full_wpo_on_official) {
-      arflags = [ "/LTCG" ]
+      if (use_incremental_ltcg) {
+        arflags = [ "/LTCG:INCREMENTAL" ]
+      } else {
+        arflags = [ "/LTCG" ]
+      }
     }
   }
 } else {
   common_optimize_on_cflags = []
   common_optimize_on_ldflags = []
 
   if (is_android) {
     # TODO(jdduke) Re-enable on mips after resolving linking
     # issues with libc++ (crbug.com/456380).
     if (current_cpu != "mipsel" && current_cpu != "mips64el") {
@@ skipping 344 matching lines @@
 
 if (is_ios || is_mac) {
   # On Mac and iOS, this enables support for ARC (automatic ref-counting).
   # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
   config("enable_arc") {
     common_flags = [ "-fobjc-arc" ]
     cflags_objc = common_flags
     cflags_objcc = common_flags
   }
 }