Factor iOS native image loading into a utility function.

ios/chrome/browser/ui/tab_switcher/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.
 
 bundle_data("resources") {
   sources = [
     "resources/tabswitcher_full_history.png",
     "resources/tabswitcher_full_history@2x.png",
     "resources/tabswitcher_full_history@3x.png",
     "resources/tabswitcher_incognito.png",
@@ skipping 134 matching lines @@
   deps = [
     "//base",
     "//components/browser_sync",
     "//components/sync",
     "//components/sync_sessions",
     "//ios/chrome/app/strings",
     "//ios/chrome/app/theme",
     "//ios/chrome/browser/browser_state",
     "//ios/chrome/browser/favicon",
     "//ios/chrome/browser/sync",
+    "//ios/chrome/browser/ui",
     "//ui/base",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "tab_model_snapshot_unittest.mm",
     "tab_switcher_model_unittest.mm",
     "tab_switcher_utils_unittest.mm",
@@ skipping 27 matching lines @@
     "//ios/chrome/test/app:test_support",
     "//ios/chrome/test/earl_grey:test_support",
     "//ios/third_party/earl_grey",
     "//ui/base",
   ]
   libs = [
     "UIKit.framework",
     "XCTest.framework",
   ]
 }