Enable precompiled headers for Blink on Windows

build/config/pch.gni

+# 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() {
+  # 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.
+  if (is_official_build || use_goma) {
+    disable_precompiled_headers = true
+  } else {
+    disable_precompiled_headers = false
+  }
+}