MD WebUI: Center search field correctly inside cr-toolbar

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

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-media-query/iron-media-query.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_toolbar/cr_toolbar_search_field.html">
 
 <dom-module id="cr-toolbar">
   <template>
     <style>
       :host {
         color: #fff;
         display: flex;
         height: 56px;
       }
 
       h1 {
         @apply(--layout-flex);
         font-size: 123%;
         font-weight: 400;
       }
 
       #leftContent {
         align-items: center;
         display: flex;
+        position: absolute;
+        transition: opacity 100ms;
       }
 
       #leftContent paper-icon-button {
         -webkit-margin-end: 6px;
         -webkit-margin-start: 18px;
         height: 32px;
         margin-bottom: 6px;
         margin-top: 6px;
         min-width: 32px;
         padding: 6px;
         width: 32px;
       }
 
       #centeredContent {
         -webkit-padding-end: 12px;
         display: flex;
         flex: 1 1 0;
         justify-content: center;
       }
 
+      :host(:not([narrow_])) #centeredContent {
+        /*
+         * Create a 180px area on the left which the search field will never
+         * overlap (it will go off-center to avoid it).
+         */
+        margin: 0 180px;
+      }
+
       :host([narrow_]) #centeredContent {
         justify-content: flex-end;
       }
 
       :host([narrow_][showing-search_]) #leftContent {
-        display: none;
+        opacity: 0;
       }
     </style>
     <div id="leftContent">
       <paper-icon-button icon="cr:menu" on-tap="onMenuTap_">
       </paper-icon-button>
       <h1>[[pageName]]</h1>
     </div>
 
     <div id="centeredContent">
       <cr-toolbar-search-field id="search" narrow="[[narrow_]]"
           label="[[searchPrompt]]" clear-label="[[clearLabel]]"
           spinner-active="[[spinnerActive]]"
           showing-search="{{showingSearch_}}">
       </cr-toolbar-search-field>
       <iron-media-query query="(max-width: 900px)" query-matches="{{narrow_}}">
       </iron-media-query>
     </div>
   </template>
   <script src="cr_toolbar.js"></script>
 </dom-module>