MD Settings: Change some instances of "Not valid" to "Not a valid web address".

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">
@@ skipping 110 matching lines @@
               <controlled-radio-button id="custom-input" class="list-item"
                   name="false" pref="[[prefs.homepage_is_newtabpage]]"
                   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" pref="{{prefs.homepage}}"
                     label="$i18n{enterCustomWebAddress}"
                     can-tab="[[!prefs.homepage_is_newtabpage.value]]"
                     no-extension-indicator
-                    error-message="$i18n{notValid}"
+                    error-message="$i18n{notValidWebAddress}"
                     on-input="validate_" invalid="{{isHomeUrlInvalid_}}">
                 </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_">
@@ skipping 55 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>