Add gn option 'is_lto' to enable Link Time Optimization builds.

build/toolchain/gcc_toolchain.gni

 # 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/nacl/config.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.gni")
 
 # This value will be inherited in the toolchain below.
-if (is_cfi) {
+if (is_lto) {
   concurrent_links =
       exec_script("get_concurrent_links.py", [ "--lto" ], "value")
 } else {
   concurrent_links = exec_script("get_concurrent_links.py", [], "value")
 }
 
 # This template defines a toolchain for something that works like gcc
 # (including clang).
 #
 # It requires the following variables specifying the executables to run:
@@ skipping 187 matching lines @@
       depsformat = "gcc"
       description = "ASM {{output}}"
       outputs = [
         "$object_subdir/{{source_name_part}}.o",
       ]
     }
 
     tool("alink") {
       rspfile = "{{output}}.rsp"
       arflags = ""
-      if (is_cfi && invoker.toolchain_os != "nacl") {
+      if (is_lto && invoker.toolchain_os != "nacl") {
         gold_plugin_path = rebase_path(
                 "//third_party/llvm-build/Release+Asserts/lib/LLVMgold.so",
                 root_build_dir)
         arflags = "--plugin \"$gold_plugin_path\""
       }
 
       # This needs a Python script to avoid using simple sh features in this
       # command, in case the host does not use a POSIX shell (e.g. compiling
       # POSIX-like toolchains such as NaCl on Windows).
       ar_wrapper =
@@ skipping 226 matching lines @@
     nm = "${toolprefix}nm"
 
     forward_variables_from(invoker,
                            [
                              "toolchain_cpu",
                              "toolchain_os",
                              "use_gold",
                            ])
   }
 }