MD Settings: validate home button url input

chrome/app/settings_strings.grdp

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Settings-specific strings (included from generated_resources.grd). -->
 <grit-part>
   <!-- Main Page -->
   <if expr="chromeos">
     <message name="IDS_SETTINGS_SECONDARY_USER_BANNER" desc="Banner displayed in settings page when the user is secondary in a multi-profile session.">
       Some settings belonging to <ph name="PRIMARY_EMAIL">$1<ex>john@google.com</ex></ph> are being shared with you. These settings only affect your account when using multiple sign-in.
     </message>
   </if>
 
@@ skipping 222 matching lines @@
   </message>
   <message name="IDS_SETTINGS_CUSTOM_WEB_ADDRESS" desc="Sub label describing an empty custom web address for the Show home button setting.">
     Custom
   </message>
   <message name="IDS_SETTINGS_ENTER_CUSTOM_WEB_ADDRESS" desc="Input label for a custom web address for the Show home button setting.">
     Enter custom web address
   </message>
   <message name="IDS_SETTINGS_HOME_BUTTON_DISABLED" desc="Sub label for the Show home button setting when disabled.">
     Disabled
   </message>
+  <message name="IDS_SETTINGS_HOME_URL_NOT_VALID" desc="Text indicating that the input to the custom home url is not invalid.">
+   Not valid
+  </message>
   <message name="IDS_SETTINGS_SET_WALLPAPER" desc="Name of the control which allows the user to set the wallpaper.">
     Wallpaper
   </message>
   <message name="IDS_SETTINGS_THEMES" desc="Name of the control which allows the user to get a theme for the browser.">
     Themes
   </message>
   <if expr="is_linux and not chromeos">
     <message name="IDS_SETTINGS_SYSTEM_THEME" desc="Text of the label describing the system (GTK+) browser theme on Linux">
       GTK+
     </message>
@@ skipping 2686 matching lines @@
       System
     </message>
     <message name="IDS_SETTINGS_SYSTEM_HARDWARE_ACCELERATION_LABEL" desc="Label for the checkbox that forces Chrome to render via hardware acceleration (GPU) when available.">
       Use hardware acceleration when available
     </message>
     <message name="IDS_SETTINGS_SYSTEM_PROXY_SETTINGS_BUTTON" desc="Text for the button that changes a user's network proxy settings.">
       Change proxy settings
     </message>
   </if>
 </grit-part>