MD Settings: validate home button url input

chrome/browser/resources/settings/appearance_page/appearance_page.html

 <link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/html/md_select_css.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/classes/iron-flex-layout.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/neon-animation/neon-animatable.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-icon-button/paper-icon-button-light.html">
 <link rel="import" href="appearance_browser_proxy.html">
 <link rel="import" href="../controls/controlled_radio_button.html">
 <link rel="import" href="../controls/extension_controlled_indicator.html">
 <link rel="import" href="../controls/settings_dropdown_menu.html">
 <link rel="import" href="../controls/settings_input.html">
 <link rel="import" href="../controls/settings_radio_group.html">
 <link rel="import" href="../controls/settings_toggle_button.html">
 <link rel="import" href="../route.html">
 <link rel="import" href="../settings_page/settings_animated_pages.html">
 <link rel="import" href="../settings_page/settings_subpage.html">
 <link rel="import" href="../settings_shared_css.html">
 <link rel="import" href="../settings_vars_css.html">
 <link rel="import" href="appearance_fonts_page.html">
 
 <dom-module id="settings-appearance-page">
   <template>
     <style include="settings-shared md-select">
       .secondary-button ~ .secondary-button {
         -webkit-margin-start: 12px;
       }
 
+      #custom-input {
+        margin-bottom: 1rem;
+      }
+
       #customHomePage {
-        width: 200px;
+        --paper-input-container: {
+          width: 200px;
+        };
       }
     </style>
     <settings-animated-pages id="pages" section="appearance">
       <neon-animatable route-path="default">
 <if expr="chromeos">
         <div class="settings-box first two-line" id="wallpaperButton"
             on-tap="openWallpaperManager_" actionable
             hidden="[[!pageVisibility.setWallpaper]]">
           <div class="start">
             $i18n{setWallpaper}
@@ skipping 65 matching lines @@
           </settings-toggle-button>
         </div>
         <template is="dom-if" if="[[prefs.browser.show_home_button.value]]">
           <div class="list-frame" hidden="[[!pageVisibility.homeButton]]">
             <settings-radio-group pref="{{prefs.homepage_is_newtabpage}}">
               <controlled-radio-button class="list-item" name="true"
                   pref="[[prefs.homepage_is_newtabpage]]"
                   label="$i18n{homePageNtp}" no-extension-indicator>
               </controlled-radio-button>
               <controlled-radio-button class="list-item" name="false"
-                  pref="[[prefs.homepage_is_newtabpage]]"
+                  pref="[[prefs.homepage_is_newtabpage]]" id="custom-input"

[dpapad, 2017/03/24 01:39:31]

Nit (optional): It is more common to have the id="..." be the first attribute
(before "class=").

[scottchen, 2017/03/24 20:48:54]

Done.

                   no-extension-indicator>
                 <!-- TODO(dbeam): this can show double indicators when both
                      homepage and whether to use the NTP as the homepage are
                      managed. -->
                 <settings-input id="customHomePage"
                     no-label-float pref="{{prefs.homepage}}"
                     label="$i18n{enterCustomWebAddress}"
                     can-tab="[[!prefs.homepage_is_newtabpage.value]]"
-                    stop-keyboard-event-propagation no-extension-indicator>
+                    stop-keyboard-event-propagation no-extension-indicator

[dpapad, 2017/03/24 01:39:31]

Is settings-input used anywhere else besides the appearance-page? Asking because
I see |no-extension-indicator| here, which implies that we use settings-input
elsewhere and we do want the extension indicator. Or is it obsolete code and we
can simplify settings-input itself?

[dpapad, 2017/03/24 18:07:16]

Looking in git history revealed that at some point it was used by the
downloads_page.html, but not anymore, see
https://codereview.chromium.org/1368733002. Can we clean up settings-input to
not have any dead code (probably better in a separate CL that lands before your
fix)?

[scottchen, 2017/03/24 20:48:54]

I think since I already added stuff in settings-input, it'll be easier for me to
minify the code in settings-input first, land the bugfix, and then in a separate
CL migrate it to paper-input (and delete settings-input entirely, code-search
seems to show it's not used by anything anymore). 

+                    error-message="$i18n{homeUrlNotValid}"
+                    on-input="validate_">
                 </settings-input>
               </controlled-radio-button>
               <template is="dom-if" if="[[prefs.homepage.extensionId]]">
                 <extension-controlled-indicator
                     extension-id="[[prefs.homepage.extensionId]]"
                     extension-can-be-disabled="[[
                         prefs.homepage.extensionCanBeDisabled]]"
                     extension-name="[[prefs.homepage.controlledByName]]"
                     on-disable-extension="onDisableExtension_">
                 </extension-controlled-indicator>
@@ skipping 54 matching lines @@
             associated-control="[[$$('#customize-fonts-subpage-trigger')]]"
             page-title="$i18n{customizeFonts}">
           <settings-appearance-fonts-page prefs="{{prefs}}">
           </settings-appearance-fonts-page>
         </settings-subpage>
       </template>
     </settings-animated-pages>
   </template>
   <script src="appearance_page.js"></script>
 </dom-module>