MD Settings: settings-dropdown-menu, restore "Enter" key behavior.

chrome/browser/resources/settings/controls/settings_dropdown_menu.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-dropdown-menu/paper-dropdown-menu.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-listbox/paper-listbox.html">
 <link rel="import" href="/controls/pref_control_behavior.html">
 <link rel="import" href="/i18n_setup.html">
 <link rel="import" href="/prefs/pref_util.html">
 <link rel="import" href="/settings_shared_css.html">
 
 <dom-module id="settings-dropdown-menu">
   <template>
     <style include="settings-shared">
     </style>
     <paper-dropdown-menu id="dropdownMenu" label="[[menuLabel_]]"
         on-iron-select="onSelect_" no-label-float$="[[noLabelFloat]]"
         disabled="[[shouldDisableMenu_(disabled, menuOptions.*)]]">
       <paper-listbox class="dropdown-content" selected="{{selected_}}"
           attr-for-selected="data-value">
         <template is="dom-repeat" items="[[menuOptions]]" initial-count="5">
           <!--TODO(dpapad): Use <button class="paper-item">..</button> once it
               lands in paper-item-shared-styles.html-->
-          <div class="dropdown-item" role="option" data-value$="[[item.value]]">
+          <button class="dropdown-item" role="option" data-value$="[[item.value]]">

[stevenjb, 2016/07/12 22:46:57]

Should we change to button for all paper-dropdown-menu cases? Maybe not in this
CL, but it would be good to use a consistent pattern.

Also, my understanding is that <div> is considerably cheaper than <button>, I
assume there isn't an easy way to decorate the <div> to get the behavior we
want?

[dpapad, 2016/07/12 22:56:09]

I agree that we should change it to something that works for all
paper-dropdown-menus, I am just addressing settings-dropdown-menu only in this
CL.

Regarding performance, my measurements showed that both <button> and <div> are
much faster than <paper-item>. I did not observe a noticeable difference in
performance between <div> and <button>.

[Dan Beam, 2016/07/12 23:00:27]

80 col wrap

[dpapad, 2016/07/12 23:18:02]

Done.

[michaelpg, 2016/07/12 23:44:22]

wrap

[dpapad, 2016/07/13 01:01:07]

Already done.

             [[item.name]]
-          </div>
+          </button>
         </template>
-        <div class="dropdown-item" role="option"
+        <button class="dropdown-item" role="option"
             data-value$="[[notFoundValue_]]"
             hidden$="[[!isSelectedNotFound_(selected_)]]">
           $i18n{custom}
-        </div>
+        </button>
       </paper-listbox>
     </paper-dropdown-menu>
   </template>
   <script src="settings_dropdown_menu.js"></script>
 </dom-module>