[GN]: Enable PCH for WebKit on Mac.

third_party/WebKit/Source/BUILD.gn

 # Copyright 2014 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("config.gni")
 if (is_clang) {
   import("//build/config/clang/clang.gni")
 }
 
 visibility = [ "//third_party/WebKit/*" ]
 
 # features ---------------------------------------------------------------------
 
 config("features") {
   defines = feature_defines_list
 }
 
 # inside_blink -----------------------------------------------------------------
 
 config("inside_blink") {
   defines = [
     "BLINK_IMPLEMENTATION=1",
     "INSIDE_BLINK",
   ]
 }
 
 # config -----------------------------------------------------------------------
 
 config("config") {
-  include_dirs = [ ".", ".." ]
+  include_dirs = [
+    ".",
+    "..",
+  ]
 
   cflags = []
   defines = []
 
   if (is_win) {
     cflags += [
       "/wd4305",  # Truncation from 'type1' to 'type2'.
       "/wd4324",  # Struct padded due to declspec(align).
       "/wd4714",  # Function marked forceinline not inlined.
       "/wd4800",  # Value forced to bool.
@@ skipping 35 matching lines @@
 
 # The follow configs apply to all targets except for unit tests, which rely on
 # static initializers.
 config("non_test_config") {
   cflags = []
 
   if (is_clang) {
     cflags += [ "-Wglobal-constructors" ]
   }
 }
+
+if (is_mac) {
+  # This sets up precompiled headers for Mac.
+  config("mac_precompiled_headers") {
+    precompiled_header = rebase_path("build/mac/Prefix.h", root_build_dir)
+    precompiled_source = "//third_party/WebKit/Source/build/mac/Prefix.h"
+  }
+}