MD Settings: Update some dialogs to accept Enter key.

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

Index: chrome/browser/resources/settings/on_startup_page/startup_url_dialog.html
diff --git a/chrome/browser/resources/settings/on_startup_page/startup_url_dialog.html b/chrome/browser/resources/settings/on_startup_page/startup_url_dialog.html
index a235d6af3dfa4fa2454abbbac91f426eb1f6ce46..a8c0d87a527299e8aa9424665fcede3c065d1f1b 100644
--- a/chrome/browser/resources/settings/on_startup_page/startup_url_dialog.html
+++ b/chrome/browser/resources/settings/on_startup_page/startup_url_dialog.html
@@ -1,5 +1,6 @@
 <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">
@@ -14,6 +15,8 @@
         <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">