Move the cros_* GN args to a .gni file and add a bootstrap cros toolchain.

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/toolchain.gni")
 
@@ skipping 19 matching lines @@
   # TODO(sebmarchand): Add support for the PGU (update) phase.
   chrome_pgo_phase = 0
 
   # 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 || target_cpu == "x64") {
     full_wpo_on_official = true
   } else {
     full_wpo_on_official = false
   }
+
+  # use_debug_fission: whether to use split DWARF debug info
+  # files. This can reduce link time significantly, but is incompatible
+  # 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"
 }
 
 declare_args() {
   # Whether to use the gold linker from binutils instead of lld or bfd.
   use_gold = !use_lld && !(is_chromecast && is_linux &&
                            (current_cpu == "arm" || current_cpu == "mipsel")) &&
              ((is_linux && (current_cpu == "x64" || current_cpu == "x86" ||
                             current_cpu == "arm" || current_cpu == "mipsel")) ||
               (is_android && (current_cpu == "x86" || current_cpu == "x64" ||
                               current_cpu == "arm")))
@@ skipping 15 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
   }
 }