Extract TabModelObservers from tab_model.mm to its own file.

ios/chrome/browser/tabs/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.
 
 source_set("tabs") {
   sources = [
     "tab.h",
     "tab_delegate.h",
     "tab_dialog_delegate.h",
     "tab_headers_delegate.h",
@@ skipping 101 matching lines @@
     "CoreLocation.framework",
     "UIKit.framework",
   ]
 }
 
 source_set("tabs_internal_arc") {
   sources = [
     "legacy_tab_helper.h",
     "legacy_tab_helper.mm",
     "tab_model_list.mm",
+    "tab_model_observers.h",
+    "tab_model_observers.mm",
   ]
   deps = [
     ":tabs",
     "//base",
     "//ios/chrome/browser",
     "//ios/chrome/browser/browser_state",
     "//ios/web",
   ]
   libs = [ "Foundation.framework" ]
   configs += [ "//build/config/compiler:enable_arc" ]
@@ skipping 45 matching lines @@
     ":tabs",
     ":tabs_internal",
     "//base",
     "//ios/chrome/browser/browser_state:test_support",
     "//ios/chrome/browser/sessions:test_support",
     "//ios/web:test_support",
     "//testing/gtest",
   ]
   configs += [ "//build/config/compiler:enable_arc" ]
 }