Migrate legacy HTML import for cr_search_field_behavior.html

Chromium change:

https://chromium.googlesource.com/chromium/src.git/+/72af65941757284c13927c36a582426144b4043a

commit 72af65941757284c13927c36a582426144b4043a
Author: rbpotter <rbpotter@chromium.org>
Date:   Thu Mar 25 19:17:57 2021 +0000

    WebUI: Check in Polymer 3 cr_search_field

    Bug: 1184053

ui/webui/resources/br_elements/br_toolbar/BUILD.gn

@@ -12,5 +12,4 @@ polymer_modulizer("br_toolbar_search_field") {
   js_file = "br_toolbar_search_field.js"
   html_file = "br_toolbar_search_field.html"
   html_type = "dom-module"
-  auto_imports = [ "ui/webui/resources/cr_elements/cr_search_field/cr_search_field_behavior.html|CrSearchFieldBehavior" ]
 }

ui/webui/resources/br_elements/br_toolbar/br_toolbar_search_field.html

@@ -1,6 +1,5 @@
 
 <link rel="import" href="chrome://resources/html/polymer.html">
-<link rel="import" href="chrome://resources/cr_elements/cr_search_field/cr_search_field_behavior.html">
 
 <dom-module id="br-toolbar-search-field">
   <template>

ui/webui/resources/br_elements/br_toolbar/br_toolbar_search_field.js

@@ -3,6 +3,8 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this file,
 // you can obtain one at http://mozilla.org/MPL/2.0/.
 
+import {CrSearchFieldBehavior} from 'chrome://resources/cr_elements/cr_search_field/cr_search_field_behavior.js';
+
 Polymer({
   is: 'br-toolbar-search-field',