Add precompiled headers to GN build for large targets.

build/toolchain/win/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.
 
 declare_args() {
   # Path to the directory containing the VC binaries for the right
   # combination of host and target architectures. Currently only the
   # 64-bit host toolchain is supported, with either 32-bit or 64-bit targets.
   # If vc_bin_dir is not specified on the command line (and it normally
   # isn't), we will dynamically determine the right value to use at runtime.
@@ skipping 62 matching lines @@
 
   cl = invoker.cl
 
   toolchain(target_name) {
     # Make these apply to all tools below.
     lib_switch = ""
     lib_dir_switch = "/LIBPATH:"
 
     tool("cc") {
       rspfile = "{{output}}.rsp"
-
-      # TODO(brettw) enable this when GN support in the binary has been rolled.
-      #precompiled_header_type = "msvc"
+      precompiled_header_type = "msvc"
       pdbname = "{{target_out_dir}}/{{target_output_name}}_c.pdb"
       command = "ninja -t msvc -e $env -- $cl /nologo /showIncludes /FC @$rspfile /c {{source}} /Fo{{output}} /Fd$pdbname"
       depsformat = "msvc"
       description = "CC {{output}}"
       outputs = [
         "{{target_out_dir}}/{{target_output_name}}/{{source_name_part}}.obj",
       ]
       rspfile_content = "{{defines}} {{include_dirs}} {{cflags}} {{cflags_c}}"
     }
 
     tool("cxx") {
       rspfile = "{{output}}.rsp"
-
-      # TODO(brettw) enable this when GN support in the binary has been rolled.
-      #precompiled_header_type = "msvc"
+      precompiled_header_type = "msvc"
 
       # The PDB name needs to be different between C and C++ compiled files.
       pdbname = "{{target_out_dir}}/{{target_output_name}}_cc.pdb"
       command = "ninja -t msvc -e $env -- $cl /nologo /showIncludes /FC @$rspfile /c {{source}} /Fo{{output}} /Fd$pdbname"
       depsformat = "msvc"
       description = "CXX {{output}}"
       outputs = [
         "{{target_out_dir}}/{{target_output_name}}/{{source_name_part}}.obj",
       ]
       rspfile_content = "{{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}}"
@@ skipping 133 matching lines @@
   }
   msvc_toolchain("clang_x64") {
     environment = "environment.x64"
     current_cpu = "x64"
     prefix = rebase_path("//third_party/llvm-build/Release+Asserts/bin",
                          root_build_dir)
     cl = "${goma_prefix}$prefix/clang-cl.exe"
     is_clang = true
   }
 }