MD Settings: Add placeholder text when adding/editing a content exception.

chrome/browser/resources/settings/site_settings/add_site_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/html/web_ui_listener_behavior.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="../settings_shared_css.html">
 <link rel="import" href="constants.html">
 <link rel="import" href="site_settings_behavior.html">
 
 <dom-module id="add-site-dialog">
   <template>
     <style include="settings-shared">
       #incognito {
         margin-bottom: 2px;
         margin-top: 15px;
       }
     </style>
     <dialog is="cr-dialog" id="dialog" close-text="$i18n{close}">
       <div class="title">$i18n{addSiteTitle}</div>
       <div class="body">
         <iron-a11y-keys id="keys" keys="enter"
             on-keys-pressed="onSubmit_"></iron-a11y-keys>
         <paper-input id="site" always-float-label label="$i18n{addSite}"
+            placeholder="$i18n{addSiteExceptionPlaceholder}"
             value="{{site_}}" on-input="validate_"></paper-input>
         <paper-checkbox id="incognito">
           $i18n{incognitoSiteOnly}
         </paper-checkbox>
       </div>
       <div class="button-container">
         <paper-button class="cancel-button" on-tap="onCancelTap_">
           $i18n{cancel}
         </paper-button>
         <paper-button class="action-button" id="add"
             on-tap="onSubmit_" disabled>
           $i18n{add}
         </paper-button>
       </div>
     </dialog>
   </template>
   <script src="add_site_dialog.js"></script>
 </dom-module>