Add the concept of a source set to GN.

tools/gn/secondary/chrome/BUILD.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.
 
 import("//crypto/ssl/flags.gni")
 import("//tools/grit/grit_rule.gni")
 
 executable("chrome") {
   sources = [
     "app/breakpad_field_trial_win.cc",
@@ skipping 78 matching lines @@
     # common/net uses NSS.
     # TODO(brettw) have a better way to express this without having to do
     # "if (use_nss)" everywhere.
     configs += "//third_party/nss:nss_linux_config"
   }
   deps = [
   ]
 }
 
 # TODO(brettw) move to browser/devtools/BUILD.gn
-static_library("debugger") {
+source_set("debugger") {
   sources = [
     "browser/devtools/adb/android_rsa.cc",
     "browser/devtools/adb/android_rsa.h",
     "browser/devtools/adb/android_usb_device.cc",
     "browser/devtools/adb/android_usb_device.h",
     "browser/devtools/adb/android_usb_socket.cc",
     "browser/devtools/adb/android_usb_socket.h",
     "browser/devtools/adb_client_socket.cc",
     "browser/devtools/adb_client_socket.h",
     "browser/devtools/adb_web_socket.cc",
@@ skipping 50 matching lines @@
   #   defines = [ "DEBUG_DEVTOOLS=1" ]
   # }
   # TODO(brettw) this may be required on Windows:
   # if (is_win) {
   #   TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
   #   'msvs_disabled_warnings': [ 4267, ]
   # }
 
 }
 
-static_library("plugin") {
+source_set("plugin") {
   external = true
 }
 
-static_library("renderer") {
+source_set("renderer") {
   external = true
 }
 
-static_library("utility") {
+source_set("utility") {
   external = true
 }
 
 
 # Credits ----------------------------------------------------------------------
 
 about_credits_file = "$root_gen_dir/about_credits.html"
 build_relative_about_credits_file =
   rebase_path(about_credits_file, ".", root_build_dir)
 
@@ skipping 110 matching lines @@
     ":theme_resources_gen",
     #"//ui:ui_resources",
   ]
 }
 
 # TODO(brettw) rename "app_theme_resources" or something when we don't support
 # GYP any more. This name is required to match the GYP build.
 grit("theme_resources_gen") {
   source = "app/theme/theme_resources.grd"
 }