Add support for 32-bit and 64-bit Windows compiles in GN.

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 25 matching lines @@
   # ASH is enabled.
   # TODO(brettw) this should be moved out of the main build config file.
   use_ash = false
   # Aura is enabled.
   # TODO(brettw) this should be moved out of the main build config file.
   use_aura = false
   # Ozone is enabled.
   # TODO(brettw) this should be moved out of the main build config file.
   use_ozone = false
 
+  # 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
+
   # Set to true on the command line when invoked by GYP. Build files can key
   # off of this to make any GYP-output-specific changes to the build.
   is_gyp = false
 }
 
 # =============================================================================
 # OS DEFINITIONS
 # =============================================================================
 #
 # We set these various is_FOO booleans for convenience in writing OS-based
@@ skipping 75 matching lines @@
   is_nacl = false
   is_posix = true
   is_win = false
 }
 
 # =============================================================================
 # CPU ARCHITECTURE
 # =============================================================================
 
 if (is_win) {
-  # Always use 32-bit on Windows, even when compiling on a 64-bit host OS.
-  # TODO(brettw) when we support 64-bit cross-compiles, we probably need to
-  # set a build arg in the toolchain to disable this override.
-  cpu_arch = "ia32"
+  # 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"
+  }
 }
 
 # =============================================================================
 # SOURCES FILTERS
 # =============================================================================
 #
 # These patterns filter out platform-specific files when assigning to the
 # sources variable. The magic variable |sources_assignment_filter| is applied
 # to each assignment or appending to the sources variable and matches are
 # automatcally removed.
@@ skipping 209 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) {
-  host_toolchain = "//build/toolchain/win:32"
+  if (cpu_arch == "x64") {
+    host_toolchain = "//build/toolchain/win:64"
+  } else if (cpu_arch == "x86") {
+    host_toolchain = "//build/toolchain/win:32"
+  }
   set_default_toolchain(host_toolchain)
 } else if (is_linux) {
   host_toolchain = "//build/toolchain/linux:host"
   if (cpu_arch == "arm" && build_cpu_arch != "arm") {
     # Special toolchain for ARM cross-compiling.
     set_default_toolchain("//build/toolchain/linux:arm-cross-compile")
   } else {
     # Use whatever GCC is on the current platform.
     set_default_toolchain(host_toolchain)
   }
 } else if (is_mac) {
   host_toolchain = "//build/toolchain/mac:clang"
   set_default_toolchain(host_toolchain)
 }