Enable precompiled headers for Blink on Windows

build/config/pch.gni

Index: build/config/pch.gni
diff --git a/build/config/pch.gni b/build/config/pch.gni
new file mode 100644
index 0000000000000000000000000000000000000000..4aa539487332b083adc2e23458b79d9158f3382a
--- /dev/null
+++ b/build/config/pch.gni
@@ -0,0 +1,10 @@
+# Copyright 2016 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.
+
+declare_args() {
+  # Normally we try to decide whether to use precompiled headers or
+  # not based on the other build arguments, but in some cases it is
+  # easiest to force them off explicitly.
+  disable_precompiled_headers = false
+}