MD Settings: validate home button url input

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

Index: chrome/browser/resources/settings/appearance_page/appearance_page.html
diff --git a/chrome/browser/resources/settings/appearance_page/appearance_page.html b/chrome/browser/resources/settings/appearance_page/appearance_page.html
index f741a59b07b4c642e7488d0fc6ba9e3e1ba31871..c428cff20a06954f309e154ac583a503d62d5090 100644
--- a/chrome/browser/resources/settings/appearance_page/appearance_page.html
+++ b/chrome/browser/resources/settings/appearance_page/appearance_page.html
@@ -26,8 +26,14 @@
         -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">
@@ -113,7 +119,7 @@
                   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
@@ -122,7 +128,9 @@
                     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]]">