Convert source sets to static libraries.

BUILD.gn

 # Copyright 2016 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_overrides/v8.gni")
 import("//testing/test.gni")
 import("pdfium.gni")
 
 config("pdfium_common_config") {
   cflags = []
@@ skipping 129 matching lines @@
     ]
   }
 
   if (pdfium_bundle_freetype) {
     deps += [ "third_party:fx_freetype" ]
   } else {
     libs += [ "freetype" ]
   }
 }
 
-source_set("test_support") {
+static_library("test_support") {
   testonly = true
   sources = [
     "testing/fx_string_testhelpers.cpp",
     "testing/fx_string_testhelpers.h",
     "testing/test_support.cpp",
     "testing/test_support.h",
     "testing/utils/path_service.cpp",
   ]
   deps = [
     ":pdfium",
@@ skipping 1428 matching lines @@
 }
 
 if (pdf_is_standalone) {
   source_set("samples") {
     testonly = true
     deps = [
       "//samples",
     ]
   }
 }