Reland of Update the nacl_bootstrap GN code to work in ChromeOS chroot builds.

src/trusted/service_runtime/linux/BUILD.gn

 # Copyright (c) 2014 The Native Client Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # We use a special toolchain to build nacl_helper_bootstrap.  This
 # seems a bit excessive for compiling one source file.  But it is
 # the most reasonable way to instantiate the configs with different
 # settings such as use_gold=false.  The minimal set of configs used
 # for that file (see below) will sometimes try to use the switch
 # -gsplit-dwarf under use_gold=true.  But without some other configs
 # that add a -B option pointing at a custom-built binutils, on some
 # builders (e.g. on older bots) Clang will fail with -gsplit-dwarf.
 # Fortunately, -gsplit-dwarf is pointless for this build anyway
 # since there is just one small file.  And we're not actually using
 # Gold for the link, so use_gold=true is a false statement and might
 # cause other problems.
 
-here = get_label_info(".", "dir")
-bootstrap_toolchain = here + "/toolchain:nacl_bootstrap_" + target_cpu
+# TODO(dpranke): There needs to be a better way to determine if
+# we are using the custom CrOS toolchains, but for now we check is_clang,
+# which is the same way we check in //build/config/BUILDCONFIG.gn.
+_is_cros_target_toolchain = is_chromeos && !is_clang
+if (target_os == "chromeos") {
+  import("//build/toolchain/cros_toolchain.gni")
+}
+
+if (_is_cros_target_toolchain) {
+  bootstrap_toolchain = "//build/toolchain/cros:nacl_bootstrap"
+} else {
+  here = get_label_info(".", "dir")
+  bootstrap_toolchain = here + "/toolchain:nacl_bootstrap_" + target_cpu
+}
 
 if (current_toolchain == bootstrap_toolchain) {
   bootstrap_lib_path = "$target_out_dir/libnacl_bootstrap_lib.a"
   bootstrap_raw_path = "$target_out_dir/nacl_bootstrap_raw"
   bootstrap_exe_path = "$root_out_dir/nacl_helper_bootstrap"
 
   linker_script_path = rebase_path("nacl_bootstrap.x")
 
   if (current_cpu == "x64") {
     nacl_reserve_top = "0x0"
     linker_emulation = "elf_x86_64"
   }
   if (current_cpu == "x86") {
     nacl_reserve_top = "0x40000000"
     linker_emulation = "elf_i386"
   }
   if (current_cpu == "arm") {
     nacl_reserve_top = "0x40002000"
     linker_emulation = "armelf_linux_eabi"
   }
   if (current_cpu == "mipsel") {
     nacl_reserve_top = "0x40008000"
     linker_emulation = "elf32ltsmip"
   }
 
-  if (is_android) {
+  if (_is_cros_target_toolchain) {
+    compiler = cros_target_cxx
+  } else if (is_android) {
     compiler = "g++"
   } else if (current_cpu == "arm") {
     compiler = "arm-linux-gnueabihf-g++"
   } else {
     compiler = "g++"
   }
 
-  if (target_os == "chromeos" && defined(cros_target_cxx) &&
-      cros_target_cxx != "") {
-    compiler = cros_target_cxx
-  }
-
   static_library("nacl_bootstrap_lib") {
     # This code has very specific requirements related to its static
     # linking and minimal libc use.  It cannot support the normal variety
     # of build configs that the rest of the code is built with.  So it
     # resets the list of configsto empty, and then builds up from scratch
     # using only the basics that work here.
     configs = []
     configs += [
       "//build/config/compiler:compiler_cpu_abi",
       "//build/config/compiler:runtime_library",
@@ skipping 92 matching lines @@
   copy("bootstrap") {
     bootstrap = ":munge_nacl_helper_bootstrap($bootstrap_toolchain)"
 
     bootstrap_out_dir = get_label_info(bootstrap, "root_out_dir")
 
     deps = [ bootstrap ]
     sources = [ "$bootstrap_out_dir/nacl_helper_bootstrap" ]
     outputs = [ "$root_out_dir/nacl_helper_bootstrap" ]
   }
 }