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

ios/chrome/common/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("common") {
   sources = [
     "channel_info.h",
     "channel_info.mm",
     "material_timing.h",
     "material_timing.mm",
     "string_util.h",
     "string_util.mm",
   ]
 
   deps = [
     "//base",
     "//components/version_info",
     "//ios/chrome/common/app_group:main_app",
     "//ios/chrome/common/physical_web:physical_web",
   ]
 
-  libs = [
-    "CoreGraphics.framework",
-    "Foundation.framework",
+  libs = [ "QuartzCore.framework" ]
+}
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "string_util_unittest.mm",
+  ]
+  deps = [
+    ":common",
+    "//base",
+    "//testing/gtest",
   ]
 }