Add precompiled headers to GN build for large targets.

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")
 if (current_cpu == "arm") {
   import("//build/config/arm.gni")
 }
 if (current_cpu == "mipsel" || current_cpu == "mips64el") {
@@ skipping 90 matching lines @@
   # below.
   if (is_win) {
     # Windows compiler flags setup.
     # -----------------------------
     cflags += [
       "/Gy",  # Enable function-level linking.
       "/GS",  # Enable buffer security checking.
       "/FS",  # Preserve previous PDB behavior.
     ]
 
+    # Force C/C++ mode for the given GN detected file type. This is necessary
+    # for precompiled headers where the same source file is compiled in both
+    # modes.
+    cflags_c += [ "/TC" ]
+    cflags_cc += [ "/TP" ]
+
     # Building with Clang on Windows is a work in progress and very
     # experimental. See crbug.com/82385.
     # Keep this in sync with the similar block in build/common.gypi
     if (is_clang) {
       cflags += [
         # Many files use intrinsics without including this header.
         # TODO(hans): Fix those files, or move this to sub-GYPs.
         "/FIIntrin.h",
       ]
 
@@ skipping 1142 matching lines @@
       cflags += [ "-gsplit-dwarf" ]
     }
   }
 }
 
 config("no_symbols") {
   if (!is_win) {
     cflags = [ "-g0" ]
   }
 }