Update from https://crrev.com/304418

build/config/BUILDCONFIG.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.
 
 # =============================================================================
 # BUILD FLAGS
 # =============================================================================
 #
 # This block lists input arguments to the build, along with their default
 # values. GN requires listing them explicitly so it can validate input and have
@@ skipping 16 matching lines @@
 
   # Component build.
   is_component_build = false
   # Debug build.
   is_debug = true
 
   # Set to true when compiling with the Clang compiler. Typically this is used
   # to configure warnings.
   is_clang = (os == "mac" || os == "ios" || os == "linux" || os == "chromeos")
 
-  # Forces a 64-bit build on Windows. Does nothing on other platforms. Normally
-  # we build 32-bit on Windows regardless of the current host OS bit depth.
-  # Setting this flag will override this logic and generate 64-bit toolchains.
-  #
-  # Normally this would get set automatically when you specify a target using
-  # the 64-bit toolchain. You can also set this on the command line to convert
-  # the default toolchain to 64-bit.
-  force_win64 = false
-
   # Selects the desired build flavor. Official builds get additional
   # processing to prepare for release. Normally you will want to develop and
   # test with this flag off.
   is_official_build = false
 
   # Select the desired branding flavor. False means normal Chromium branding,
   # true means official Google Chrome branding (requires extra Google-internal
   # resources).
   is_chrome_branded = false
 
@@ skipping 111 matching lines @@
   is_posix = true
   is_win = false
 }
 
 is_desktop_linux = is_linux && !is_chromeos
 
 # =============================================================================
 # CPU ARCHITECTURE
 # =============================================================================
 
-if (is_win) {
-  # Always use 32-bit on Windows, even when compiling on a 64-bit host OS,
-  # unless the override flag is specified.
-  if (force_win64) {
-    cpu_arch = "x64"
-  } else {
-    cpu_arch = "x86"
-  }
-}
-
 if (is_android) {
   # TODO(cjhopman): enable this assert once bots are updated to not set
   # cpu_arch.
   #assert(cpu_arch == build_cpu_arch, "Android device target architecture should
   #    be set with 'target_arch', not 'cpu_arch'")
   cpu_arch = target_arch
 }
 
 # =============================================================================
 # SOURCES FILTERS
@@ skipping 291 matching lines @@
 # ==============================================================================
 # TOOLCHAIN SETUP
 # ==============================================================================
 #
 # Here we set the default toolchain, as well as the variable host_toolchain
 # which will identify the toolchain corresponding to the local system when
 # doing cross-compiles. When not cross-compiling, this will be the same as the
 # default toolchain.
 
 if (is_win) {
-  # TODO(brettw) name the toolchains the same as cpu_arch as with Linux below
-  # to eliminate these conditionals.
-  if (build_cpu_arch == "x64") {
+  # On windows we use the same toolchain for host and target by default.
+  # TODO(dpranke): rename the toolchains to x64 and x86 to match cpu_arch.
+  if (cpu_arch == "x64") {
     host_toolchain = "//build/toolchain/win:64"
-  } else if (build_cpu_arch == "x86") {
+  } else if (cpu_arch == "x86") {
     host_toolchain = "//build/toolchain/win:32"
   }
-
-  if (cpu_arch == "x64") {
-    set_default_toolchain("//build/toolchain/win:64")
-  } else if (cpu_arch == "x86") {
-    set_default_toolchain("//build/toolchain/win:32")
-  }
+  set_default_toolchain("$host_toolchain")
 } else if (is_android) {
   # Use clang for the x86/64 Linux host builds.
   if (build_cpu_arch == "x86" || build_cpu_arch == "x64") {
     host_toolchain = "//build/toolchain/linux:clang_$build_cpu_arch"
   } else {
     host_toolchain = "//build/toolchain/linux:$build_cpu_arch"
   }
   set_default_toolchain("//build/toolchain/android:$cpu_arch")
 } else if (is_linux) {
   if (is_clang) {
@@ skipping 195 matching lines @@
       if (defined(invoker.output_extension)) { output_extension = invoker.output_extension }
       if (defined(invoker.output_name)) { output_name = invoker.output_name }
       if (defined(invoker.public)) { public = invoker.public }
       if (defined(invoker.public_configs)) { public_configs = invoker.public_configs }
       if (defined(invoker.public_deps)) { public_deps = invoker.public_deps }
       if (defined(invoker.sources)) { sources = invoker.sources }
       if (defined(invoker.visibility)) { visibility = invoker.visibility }
     }
   }
 }