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;
+      }
     </style>
     <div id="leftContent">
       <template is="dom-if" if="[[showMenu]]">
         <paper-icon-button id="menuButton" icon="cr:menu" on-tap="onMenuTap_"
-            title="[[menuLabel]]">
+            title="[[titleIfNotShowingMenuPromo_(menuLabel,
+                                                 showingMenuPromo_)]]">
         </paper-icon-button>
+        <template is="dom-if" if="[[showingMenuPromo_]]">
+          <paper-tooltip for="menuButton" position="bottom" offset="20"
+              manual-mode fit-to-visible-bounds>
+            [[menuPromo]]
+            <iron-icon icon="cr:clear" on-tap="onMenuPromoCloseTap_">

[tsergeant, 2016/08/25 04:51:00]

Should this be a paper-icon-button[-light]? Or just a native <button> with an
<iron-icon> inside it? Something to indicate to screen readers (and everyone
else, with `cursor: pointer`) that this is clickable.

[Dan Beam, 2016/09/09 03:12:08]

I asked Alan both of those questions: he wanted default cursor (not hand), and
we agreed that having a ripple didn't make sense for vanishing UI.  I'm happy to
push harder on either if you'd like.

Also note: we can set up the right aria-role and tabindex if you'd like
separately from whether it's a <paper-*> thing.

I'm really just trying not to add weight to the page.  I was even considering
not using <paper-tooltip> (and just a div with similar styles), and maybe even a
unicode X like in downloads.

I read that the toolbar currently accounts for like 7% of load, but feel these
were a) micro optimizations, b) not adding much, though I think I am adding the
first use of paper-tooltip but discarding much of the benefit with
[manual-mode].

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

Happy to go with this for the lightweight button.

My main concern is for screenreader users. I think it's fine for the promo to
appear not be in the document tab order. I don't think we need to draw people's
attention to it like that. However, if a screenreader user does navigate into
the promo tooltip, they should be able to understand what the icon is. That
probably means role="button" and an aria-label, which is a bummer since we need
to plumb through an extra string.

If you'd like to test it out on canary with a screenreader first before you make
those changes, I'd be okay with that. 

(And thanks for keeping performance in mind 😃)

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

Done.

+            </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>