Enable precompiled headers for Blink on Windows.

third_party/WebKit/Source/core/core.gni

 # 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("//build/config/chrome_build.gni")
 import("//build/split_static_library.gni")
 import("//third_party/WebKit/Source/config.gni")
 
 blink_core_output_dir = "$root_gen_dir/blink/core"
 
@@ skipping 104 matching lines @@
                              "public_deps",
                              "split_count",
                            ])
   }
 }
 set_defaults("blink_core_sources") {
   # This sets the default list of configs when the blink_core_sources target
   # is defined. The default_compiler_configs comes from BUILDCONFIG.gn and
   # is the list normally applied to static libraries and source sets.
   configs = default_compiler_configs - core_config_remove + core_config_add
+
+  # Compile each of the core sources targets with (core) precompiled header
+  # support, for lower Windows build times.
+  configs += [ "//third_party/WebKit/Source/core:blink_core_pch" ]
 }