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

build/config/mac/mac_sdk.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")
+
 declare_args() {
   # Minimum supported version of the Mac SDK.
   mac_sdk_min = "10.6"
 
   # Path to a specific version of the Mac SDKJ, not including a backslash at
   # the end. If empty, the path to the lowest version greater than or equal to
   # mac_sdk_min is used.
   mac_sdk_path = ""
 }
 
@@ skipping 15 matching lines @@
 mac_sdk_version = find_sdk_lines[1]
 if (mac_sdk_path == "") {
   # TODO(brettw) http://crbug.com/335325  when everybody moves to XCode 5 we
   # can remove the --print_sdk_path argument to find_sdk and instead just use
   # the following two lines to get the path. Although it looks longer here, it
   # saves forking a process in find_sdk.py so will be faster.
   #mac_sdk_root = "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX"
   #mac_sdk_path = mac_sdk_root + mac_sdk_version + ".sdk"
   mac_sdk_path = find_sdk_lines[0]
 }