Add precompiled headers to GN build for large targets.

build/precompile.h

 // Copyright (c) 2012 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.
 
-// Precompiled header for Chromium project on Windows, not used by
-// other build configurations. Using precompiled headers speeds the
-// build up significantly, around 1/4th on VS 2010 on an HP Z600 with 12
-// GB of memory.
-//
-// Numeric comments beside includes are the number of times they were
-// included under src/chrome/browser on 2011/8/20, which was used as a
-// baseline for deciding what to include in the PCH. Includes without
-// a numeric comment are generally included at least 5 times. It may
-// be possible to tweak the speed of the build by commenting out or
-// removing some of the less frequently used headers.
+// This file is used as a precompiled header for both C and C++ files. So
+// any C++ headers must go in the __cplusplus block below.
 
 #if defined(BUILD_PRECOMPILE_H_)
 #error You shouldn't include the precompiled header file more than once.
 #endif
 
 #define BUILD_PRECOMPILE_H_
 
 #define _USE_MATH_DEFINES
 
-// The Windows header needs to come before almost all the other
-// Windows-specific headers.
-#include <Windows.h>
-#include <dwmapi.h>
-#include <shellapi.h>
-#include <wtypes.h>  // 2
-
-// Defines in atlbase.h cause conflicts; if we could figure out how
-// this family of headers can be included in the PCH, it might speed
-// up the build as several of them are used frequently.
-/*
-#include <atlbase.h>
-#include <atlapp.h>
-#include <atlcom.h>
-#include <atlcrack.h>  // 2
-#include <atlctrls.h>  // 2
-#include <atlmisc.h>  // 2
-#include <atlsafe.h>  // 1
-#include <atltheme.h>  // 1
-#include <atlwin.h>  // 2
-*/
-
-// Objbase.h and other files that rely on it bring in [ #define
-// interface struct ] which can cause problems in a multi-platform
-// build like Chrome's. #undef-ing it does not work as there are
-// currently 118 targets that break if we do this, so leaving out of
-// the precompiled header for now.
-//#include <commctrl.h>  // 2
-//#include <commdlg.h>  // 3
-//#include <cryptuiapi.h>  // 2
-//#include <Objbase.h>  // 2
-//#include <objidl.h>  // 1
-//#include <ole2.h>  // 1
-//#include <oleacc.h>  // 2
-//#include <oleauto.h>  // 1
-//#include <oleidl.h>  // 1
-//#include <propkey.h>  // 2
-//#include <propvarutil.h>  // 2
-//#include <pstore.h>  // 2
-//#include <shlguid.h>  // 1
-//#include <shlwapi.h>  // 1
-//#include <shobjidl.h>  // 4
-//#include <urlhist.h>  // 2
-
-// Caused other conflicts in addition to the 'interface' issue above.
-// #include <shlobj.h>
-
 #include <errno.h>
 #include <fcntl.h>
-#include <limits.h>  // 4
+#include <limits.h>
 #include <math.h>
-#include <memory.h>  // 1
+#include <memory.h>
 #include <signal.h>
-#include <stdarg.h>  // 1
+#include <stdarg.h>
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <time.h>  // 4
+#include <time.h>
+
+#if defined(__cplusplus)
 
 #include <algorithm>
-#include <bitset>  // 3
+#include <bitset>
 #include <cmath>
 #include <cstddef>
-#include <cstdio>  // 3
-#include <cstdlib>  // 2
+#include <cstdio>
+#include <cstdlib>
 #include <cstring>
 #include <deque>
-#include <fstream>  // 3
+#include <fstream>
 #include <functional>
-#include <iomanip>  // 2
-#include <iosfwd>  // 2
+#include <iomanip>
+#include <iosfwd>
 #include <iterator>
 #include <limits>
 #include <list>
 #include <map>
-#include <numeric>  // 2
+#include <numeric>
 #include <ostream>
 #include <queue>
 #include <set>
 #include <sstream>
 #include <stack>
 #include <string>
 #include <utility>
 #include <vector>
+
+#endif  // __cplusplus