MD WebUI: Improve tab focus behavior for toolbar search field

ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar_search_field.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-input/iron-input.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-input/paper-input-container.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_search_field/cr_search_field_behavior.html">
 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 
 <dom-module id="cr-toolbar-search-field">
   <template>
     <style>
@@ skipping 64 matching lines @@
 
       /* Any layout, search open. */
       :host([showing-search]) {
         width: 100%;
       }
 
       :host([narrow][showing-search]) #icon {
         -webkit-margin-start: 18px;
       }
     </style>
-    <paper-icon-button id="icon" icon="cr:search"
-        title="[[label]]">
+    <paper-icon-button id="icon" icon="cr:search" title="[[label]]"
+        tabindex$="[[computeIconTabIndex_(narrow)]]">
     </paper-icon-button>
-    <paper-input-container on-search="onSearchTermSearch_"
-        on-keydown="onSearchTermKeydown_" no-label-float>
+    <paper-input-container on-search="onSearchTermSearch"
+        on-keydown="onSearchTermKeydown" on-keypress="onSearchTermKeypress_"
+        no-label-float>
       <label id="prompt" for="searchInput">[[label]]</label>
       <input is="iron-input" id="searchInput" type="search"
           on-blur="onInputBlur_" incremental></input>
     </paper-input-container>
     <paper-icon-button icon="cr:cancel" id="clearSearch"
         title="[[clearLabel]]" hidden$="[[!hasSearchText]]"
         on-tap="hideSearch_">
     </paper-icon-button>
   </template>
   <script src="cr_toolbar_search_field.js"></script>
 </dom-module>