MD Settings: Update some dialogs to accept Enter key.

chrome/browser/resources/settings/search_engines_page/search_engine_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="/search_engines_page/search_engines_browser_proxy.html">
 
 <dom-module id="settings-search-engine-dialog">
   <template>
     <style include="settings-shared"></style>
     <dialog is="cr-dialog" id="dialog">
       <div class="title">[[dialogTitle_]]</div>
       <div class="body">
+        <iron-a11y-keys keys="enter" on-keys-pressed="onActionButtonTap_">
+        </iron-a11y-keys>
         <paper-input always-float-label id="searchEngine"
             label="$i18n{searchEnginesSearchEngine}"
             error-message="$i18n{searchEnginesNotValid}"
             value="{{searchEngine_}}" on-input="validate_">
         </paper-input>
         <paper-input always-float-label id="keyword"
             label="$i18n{searchEnginesKeyword}"
             error-message="$i18n{searchEnginesNotValid}"
             value="{{keyword_}}" on-focus="validate_" on-input="validate_">
         </paper-input>
         <paper-input always-float-label id="queryUrl"
             label="$i18n{searchEnginesQueryURLExplanation}"
             error-message="$i18n{searchEnginesNotValid}"
             value="{{queryUrl_}}" on-focus="validate_" on-input="validate_"
             disabled$="[[model.urlLocked]]">
         </paper-input>
       </div>
       <div class="button-container">
         <paper-button class="cancel-button" on-tap="cancel_" id="cancel">
             $i18n{cancel}</paper-button>
         <paper-button id="actionButton" class="action-button"
             on-tap="onActionButtonTap_">
           [[actionButtonText_]]
         </paper-button>
       </div>
     </dialog>
   </template>
   <script src="search_engine_dialog.js"></script>
 </dom-module>