Add a GN flag for the Windows multi-dll build.

build/config/chrome_build.gni

+# Copyright 2015 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() {
+  # Selects the desired build flavor. Official builds get additional
+  # processing to prepare for release. Normally you will want to develop and
+  # test with this flag off.
+  # TODO(brettw) move here from BUILDCONFIG.gn when DEPS are updated.
+  #is_official_build = false
+
+  # Select the desired branding flavor. False means normal Chromium branding,
+  # true means official Google Chrome branding (requires extra Google-internal
+  # resources).
+  # TODO(brettw) move here from BUILDCONFIG.gn when DEPS are updated.
+  #is_chrome_branded = false
+
+  # Break chrome.dll into multple pieces based on process type. Only available
+  # on Windows.
+  is_multi_dll_chrome = is_win && !is_component_build
+}