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

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 32 matching lines @@
   # stack frames. Setting this flag has a large effect on the performance of the
   # generated code than just setting profiling, but gives the profiler more
   # information to analyze.
   # Requires profiling to be set to true.
   enable_full_stack_frames_for_profiling = false
 
   # When we are going to use gold we need to find it.
   # This is initialized below, after use_gold might have been overridden.
   gold_path = 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"
-
   if (is_win) {
     # Whether the VS xtree header has been patched to disable warning 4702. If
     # it has, then we don't need to disable 4702 (unreachable code warning).
     # The patch is preapplied to the internal toolchain and hence all bots.
     msvs_xtree_patched = false
   }
 
   # Omit unwind support in official builds to save space.
   # We can use breakpad for these builds.
   exclude_unwind_tables = is_chrome_branded && is_official_build
@@ skipping 1469 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
   }
 }