MD History: promote menu button to show clear browsing data in narrow mode

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-icon/iron-icon.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/polymer/v1_0/paper-tooltip/paper-tooltip.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_toolbar/cr_toolbar_search_field.html">
+<link rel="import" href="chrome://resources/cr_elements/icons.html">
 
 <dom-module id="cr-toolbar">
   <template>
     <style>
       :host {
         --cr-toolbar-field-width: 580px;
         color: #fff;
         display: flex;
         height: 56px;
       }
@@ skipping 49 matching lines @@
         @apply(--cr-toolbar-right-content-wide);
       }
 
       :host([narrow_]) #centeredContent {
         justify-content: flex-end;
       }
 
       :host([narrow_][showing-search_]) #leftContent {
         opacity: 0;
       }
+
+      paper-tooltip {
+        --paper-tooltip: {
+          align-items: center;
+          display: flex;
+          font-size: 92.3%;
+          font-weight: 500;
+          padding: 6px 12px 6px 8px;
+          white-space: nowrap;
+        };
+      }
+
+      iron-icon {
+        --iron-icon-height: 20px;
+        --iron-icon-width: 20px;
+        -webkit-margin-start: 12px;
+      }
+
+      .arrow {
+        -webkit-clip-path: polygon(0 100%, 100% 100%, 50% 0);  /* Up arrow. */
+        background: inherit;
+        left: 10px;
+        height: 6px;
+        position: absolute;
+        top: -6px;
+        width: 12px;
+      }
     </style>
     <div id="leftContent">
       <template is="dom-if" if="[[showMenu]]">
         <paper-icon-button id="menuButton" icon="cr:menu" on-tap="onMenuTap_"
-            title="[[menuLabel]]">
+            title="[[titleIfNotShowMenuPromo_(menuLabel, showMenuPromo)]]">

[tsergeant, 2016/09/09 06:09:39]

Can you add `aria-label$="[[menuLabel]]"` so that the button is still meaningful
to screenreaders while the promo is showing?

[Dan Beam, 2016/09/16 00:01:24]

Done.

         </paper-icon-button>
+        <template is="dom-if" if="[[showMenuPromo]]">
+          <paper-tooltip for="menuButton" position="bottom" offset="20"
+              manual-mode fit-to-visible-bounds>
+            <div class="arrow"></div>
+            [[menuPromo]]
+            <iron-icon icon="cr:clear" on-tap="onMenuPromoCloseTap_">
+            </iron-icon>
+          </paper-tooltip>
+        </template>
       </template>
       <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>
 
     <div id="rightContent">
       <content select=".more-actions"></content>
     </div>
   </template>
   <script src="cr_toolbar.js"></script>
 </dom-module>