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

content/app/BUILD.gn

 # 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.
 
-# TODO(GYP) enable chrome_multiple_dll support
-is_chrome_multiple_dll = false
+import("//build/config/chrome_build.gni")
 
 content_app_sources = [
   "android/app_jni_registrar.cc",
   "android/app_jni_registrar.h",
   "android/child_process_service.cc",
   "android/child_process_service.h",
   "android/content_jni_onload.cc",
   "android/content_main.cc",
   "android/content_main.h",
   "android/library_loader_hooks.cc",
@@ skipping 47 matching lines @@
     "//third_party/mojo/src/mojo/edk/system",
     "//third_party/mojo/src/mojo/public/interfaces/application",
   ]
 }
 
 content_app_extra_configs = [
   "//build/config/compiler:wexit_time_destructors",
   "//content:content_implementation",
 ]
 
-if (!is_chrome_multiple_dll) {
+if (!is_multi_dll_chrome) {
   content_app_deps += [ "//content/gpu:gpu_sources" ]
 }
 
 # This includes the app sources for both the browser and child processes.
 source_set("both") {
   # Only the public target should depend on this. All other targets (even
   # internal content ones) should depend on the public one.
   visibility = [ "//content/public/app:*" ]
 
   sources = content_app_sources
   configs += content_app_extra_configs
   deps = content_app_deps
 }
 
-if (is_chrome_multiple_dll) {
+if (is_multi_dll_chrome) {
   # It doesn't make sense to do the browser/child dll split in component mode.
   assert(!is_component_build)
 
   source_set("browser") {
     visibility = [ "//content/public/app:browser" ]
 
     sources = content_app_sources
     configs += content_app_extra_configs
     deps = content_app_deps
 
-    defines += [ "CHROME_MULTIPLE_DLL_BROWSER" ]
+    defines = [ "CHROME_MULTIPLE_DLL_BROWSER" ]
   }
 
   source_set("child") {
     visibility = [ "//content/public/app:child" ]
 
     sources = content_app_sources
     configs += content_app_extra_configs
     deps = content_app_deps
 
-    defines += [ "CHROME_MULTIPLE_DLL_CHILD" ]
+    defines = [ "CHROME_MULTIPLE_DLL_CHILD" ]
   }
 }