Refactor creation of SadTabView into a tab helper object

ios/chrome/browser/web/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("//ios/web/js_compile.gni")
 
 source_set("web") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "dom_altering_lock.h",
     "dom_altering_lock.mm",
     "navigation_manager_util.h",
     "navigation_manager_util.mm",
     "network_activity_indicator_tab_helper.h",
     "network_activity_indicator_tab_helper.mm",
     "repost_form_tab_helper.h",
     "repost_form_tab_helper.mm",
+    "sad_tab_tab_helper.h",
+    "sad_tab_tab_helper.mm",
   ]
   deps = [
+    ":sad_tab_tab_helper_delegate",
     "//base",
     "//components/strings",
     "//ios/chrome/browser/ui",
     "//ios/chrome/browser/ui/alert_coordinator:alert_coordinator",
+    "//ios/chrome/browser/ui/sad_tab:sad_tab",
     "//ios/chrome/browser/ui/util:util",
     "//ios/web",
     "//ios/web:web_arc",
     "//ui/base",
   ]
 }
 
+source_set("sad_tab_tab_helper_delegate") {
+  configs += [ "//build/config/compiler:enable_arc" ]
+  sources = [
+    "sad_tab_tab_helper_delegate.h",
+  ]
+}
+
 source_set("unit_tests") {
   configs += [ "//build/config/compiler:enable_arc" ]
   testonly = true
   sources = [
     "navigation_manager_util_unittest.mm",
     "network_activity_indicator_tab_helper_unittest.mm",
     "repost_form_tab_helper_unittest.mm",
+    "sad_tab_tab_helper_unittest.mm",
   ]
   deps = [
+    ":sad_tab_tab_helper_delegate",
     ":web",
     "//base:base",
     "//base/test:test_support",
     "//components/strings:components_strings_grit",
     "//ios/chrome/browser/ui:ui",
     "//ios/chrome/test:test_support",
     "//ios/web:test_support",
     "//ios/web:web_arc",
     "//testing/gtest",
     "//ui/base:base",
@@ skipping 256 matching lines @@
     "early_page_script_perftest.mm",
   ]
   deps = [
     "//base",
     "//ios/chrome/test/base:perf_test_support",
     "//ios/web",
     "//ios/web:test_support",
   ]
   libs = [ "WebKit.framework" ]
 }