[ios] Add a BrowserListObserver interface.

ios/shared/chrome/browser/ui/browser_list/BUILD.gn

 # Copyright 2017 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("browser_list") {
   sources = [
     "browser.h",
     "browser.mm",
     "browser_list.h",
     "browser_list.mm",
+    "browser_list_observer.h",
+    "browser_list_observer.mm",
     "browser_web_state_list_delegate.h",
     "browser_web_state_list_delegate.mm",
   ]
   deps = [
     "//base",
     "//ios/chrome/browser/browser_state",
     "//ios/chrome/browser/find_in_page",
     "//ios/chrome/browser/sessions",
     "//ios/chrome/browser/ssl",
     "//ios/chrome/browser/web_state_list",
     "//ios/shared/chrome/browser/ui/commands",
   ]
   configs += [ "//build/config/compiler:enable_arc" ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "browser_list_unittest.mm",
   ]
   deps = [
     ":browser_list",
     "//base",
     "//ios/chrome/browser/browser_state:test_support",
     "//testing/gtest",
   ]
   configs += [ "//build/config/compiler:enable_arc" ]
 }