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..93bd2fedc38090e0604e974bbeed3a1e1ebbd29c
--- /dev/null
+++ b/build/config/pch.gni
@@ -0,0 +1,12 @@
+# 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.
+
+import("//build/toolchain/goma.gni")
+
+declare_args() {
+  # Precompiled header file support is by default available,
+  # but for distributed build system uses (like goma) or when
+  # doing official builds.
+  enable_precompiled_headers = !is_official_build && !use_goma
+}