Redirect Assistant settings url when Assistant disabled

Bug: 935928
Test: Local Build
Change-Id: I397d5f118d6dfbfdcd8b1b318e552780068560da
Reviewed-on: https://chromium-review.googlesource.com/c/1490114Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636124}

chrome/browser/resources/settings/route.js

@@ -264,7 +264,10 @@ cr.define('settings', function() {
     r.SEARCH = r.BASIC.createSection('/search', 'search');
     r.SEARCH_ENGINES = r.SEARCH.createChild('/searchEngines');
     // <if expr="chromeos">
-    r.GOOGLE_ASSISTANT = r.SEARCH.createChild('/googleAssistant');
+    if (loadTimeData.valueExists('assistantEnabled') &&
+        loadTimeData.getBoolean('assistantEnabled')) {
+      r.GOOGLE_ASSISTANT = r.SEARCH.createChild('/googleAssistant');
+    }
 
     r.ANDROID_APPS = r.BASIC.createSection('/androidApps', 'androidApps');
     r.ANDROID_APPS_DETAILS = r.ANDROID_APPS.createChild('/androidApps/details');

chrome/browser/ui/webui/settings/md_settings_ui.cc

@@ -339,6 +339,9 @@ MdSettingsUI::MdSettingsUI(content::WebUI* web_ui)
   html_source->AddBoolean("isDemoSession",
                           chromeos::DemoSession::IsDeviceInDemoMode());
 
+  html_source->AddBoolean("assistantEnabled",
+                          chromeos::switches::IsAssistantEnabled());
+
   // We have 2 variants of Android apps settings. Default case, when the Play
   // Store app exists we show expandable section that allows as to
   // enable/disable the Play Store and link to Android settings which is