Split //ios/chrome/browser in multiple targets.

ios/chrome/test/BUILD.gn

 # Copyright 2015 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.
 
-source_set("test") {
+# TODO(crbug.com/658242): Temporary target to help transition until all the
+# targets have been modified to depends on :test_support instead.
+group("test") {
+  testonly = true
+  public_deps = [
+    ":test_support",
+  ]
+}
+
+source_set("test_support") {
   testonly = true
   sources = [
     "block_cleanup_test.h",
     "block_cleanup_test.mm",
     "ios_chrome_scoped_testing_chrome_browser_provider.h",
     "ios_chrome_scoped_testing_chrome_browser_provider.mm",
     "ios_chrome_scoped_testing_local_state.cc",
     "ios_chrome_scoped_testing_local_state.h",
     "ios_chrome_unit_test_suite.h",
     "ios_chrome_unit_test_suite.mm",
@@ skipping 23 matching lines @@
   testonly = true
   sources = [
     "run_all_unittests.cc",
   ]
   deps = [
     ":test",
     "//base",
     "//base/test:test_support",
   ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "google_toolbox_unittest.mm",
+  ]
+  deps = [
+    "//base",
+    "//testing/gtest",
+    "//third_party/google_toolbox_for_mac",
+  ]
+}