MD Settings: fix attribute name typo to enhance a11y of JavaScript subpage...

MD Settings: fix attribute name typo to enhance a11y of JavaScript subpage button on content settings page

R=dschuyler@chromium.org
BUG=none
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2742983003
Cr-Commit-Position: refs/heads/master@{#456251}

chrome/browser/resources/settings/site_settings_page/site_settings_page.html

@@ -121,7 +121,7 @@
       <iron-icon icon="settings:input"></iron-icon>
       <div class="middle">
         $i18n{siteSettingsJavascript}
-        <div class="secondary" is="javascriptSecondary">
+        <div class="secondary" id="javascriptSecondary">
           [[defaultSettingLabel_(
               default_.javascript,
               '$i18nPolymer{siteSettingsAllowed}',