[ios] Moves find-in-page code out of Tab and into FindTabHelper.

ios/chrome/browser/find_in_page/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("find_in_page") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "find_in_page_controller.h",
     "find_in_page_controller.mm",
     "find_in_page_model.h",
     "find_in_page_model.mm",
+    "find_tab_helper.h",
+    "find_tab_helper.mm",
     "js_findinpage_manager.h",
     "js_findinpage_manager.mm",
   ]
   deps = [
     ":injected_js",
     "//base",
     "//ios/chrome/browser/web",
     "//ios/web",
   ]
   libs = [ "CoreGraphics.framework" ]
 }
 
 # TODO(crbug.com/487804): use js_compile_checked instead once the errors have
 # been fixed.
 js_compile_unchecked("injected_js") {
   sources = [
     "resources/find_in_page.js",
   ]
 }