Add the concept of a source set to GN.

tools/gn/secondary/build/config/BUILDCONFIG.gn

 # Copyright (c) 2013 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.
 
 # =============================================================================
 # BUILD FLAGS
 # =============================================================================
 #
 # This block lists input arguments to the build, along with their default
 # values. GN requires listing them explicitly so it can validate input and have
@@ skipping 144 matching lines @@
 # This is the actual set.
 set_sources_assignment_filter(sources_assignment_filter)
 
 # =============================================================================
 # BUILD OPTIONS
 # =============================================================================
 
 if (is_component_build) {
   component_mode = "shared_library"
 } else {
-  component_mode = "static_library"
+  component_mode = "source_set"
 }
 
 toolkit_uses_gtk = is_linux
 
 # =============================================================================
 # TARGET DEFAULTS
 # =============================================================================
 #
 # Set up the default configuration for every build target of the given type.
 # The values configured here will be automatically set on the scope of the
@@ skipping 83 matching lines @@
 
 set_defaults("shared_library") {
   configs = native_compiler_configs
   if (is_win) {
     configs += windows_linker_configs
   } else if (is_mac) {
     configs += "//build/config/mac:mac_dynamic_flags"
   }
 }
 
+set_defaults("source_set") {
+  configs = native_compiler_configs
+}
+
 # ==============================================================================
 # TOOLCHAIN SETUP
 # ==============================================================================
 
 if (is_win) {
   set_default_toolchain("//build/toolchain/win:32")
 } else if (is_linux) {
   set_default_toolchain("//build/toolchain/linux:gcc")
 } else if (is_mac) {
   set_default_toolchain("//build/toolchain/mac:clang")
 }