Revert of clang/win: Disable precompiled headers.

build/config/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/allocator.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/sanitizers/sanitizers.gni")
@@ skipping 420 matching lines @@
 # Add this config to your target to enable precompiled headers.
 #
 # Precompiled headers are done on a per-target basis. If you have just a couple
 # of files, the time it takes to precompile (~2 seconds) can actually be longer
 # than the time saved. On a Z620, a 100 file target compiles about 2 seconds
 # faster with precompiled headers, with greater savings for larger targets.
 #
 # Recommend precompiled headers for targets with more than 50 .cc files.
 config("precompiled_headers") {
   if (!is_official_build && !use_goma && !disable_precompiled_headers) {
-    # TODO(thakis): Reenable on clang after https://crbug.com/605570 is fixed.
-    if (is_win && !is_clang) {
+    if (is_win) {
       # This is a string rather than a file GN knows about. It has to match
       # exactly what's in the /FI flag below, and what might appear in the
       # source code in quotes for an #include directive.
       precompiled_header = "build/precompile.h"
 
       # This is a file that GN will compile with the above header. It will be
       # implicitly added to the sources (potentially multiple times, with one
       # variant for each language used in the target).
       precompiled_source = "//build/precompile.cc"
 
       # Force include the header.
       cflags = [ "/FI$precompiled_header" ]
 
       # Disable warning for "this file was empty after preprocessing". This
       # error is generated only in C mode for ANSI compatibility. It conflicts
       # with precompiled headers since the source file that's "compiled" for
       # making the precompiled header is empty.
       #
       # This error doesn't happen every time. In VS2013, it seems if the .pch
       # file doesn't exist, no error will be generated (probably MS tested this
       # case but forgot the other one?). To reproduce this error, do a build,
       # then delete the precompile.c.obj file, then build again.
       cflags_c = [ "/wd4206" ]
     } else if (is_mac) {
       precompiled_header = "build/precompile.h"
       precompiled_source = "//build/precompile.h"
     }
   }
 }