MD Settings: Update some dialogs to accept Enter key.

chrome/browser/resources/settings/on_startup_page/startup_url_dialog.html

 <link rel="import" href="chrome://resources/cr_elements/cr_dialog/cr_dialog.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
+<link rel="import" href="chrome://resources/polymer/v1_0/iron-a11y-keys/iron-a11y-keys.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-input/paper-input.html">
 <link rel="import" href="/i18n_setup.html">
 <link rel="import" href="/on_startup_page/startup_urls_page_browser_proxy.html">
 
 <dom-module id="settings-startup-url-dialog">
   <template>
     <style include="settings-shared"></style>
     <dialog is="cr-dialog" id="dialog">
       <div class="title">[[dialogTitle_]]</div>
       <div class="body">
         <paper-input always-float-label id="url"
             label="$i18n{onStartupSiteUrl}"
             value="{{url_}}" on-input="validate_">
+          <iron-a11y-keys keys="enter" on-keys-pressed="onActionButtonTap_">

[dpapad, 2017/01/25 00:47:29]

What is the benefit of using <iron-a11y-keys> instead of a simple native
keypress listener?

By reading the Docs the only benefit it seems to be cross-browser compatibility,
which is uninteresting to our Settings page.

"iron-a11y-keys provides a cross-browser interface for processing keyboard
commands."

[tommycli, 2017/01/25 01:01:47]

No very strong reason, just:

 1. Precedent... we use iron-a11y-keys in a number of other places in Settings
for key bindings.

 2. Somewhat more declarative... no need to add a (e.type == 'keypress' && e.key
!= 'Enter') check in the body of the method. Therefore we can often reuse the OK
tap handler.

[Dan Beam, 2017/01/25 01:04:52]

fwiw: i had this same question (why do you need an element and shadow DOM and
...?)

+          </iron-a11y-keys>
         </paper-input>
       </div>
       <div class="button-container">
         <div class="action-buttons">
           <paper-button class="cancel-button" on-tap="onCancelTap_"
               id="cancel">$i18n{cancel}</paper-button>
           <paper-button id="actionButton" class="action-button"
               on-tap="onActionButtonTap_">[[actionButtonText_]]</paper-button>
         </div>
       </div>
     </dialog>
   </template>
   <script src="startup_url_dialog.js"></script>
 </dom-module>