chromeos: Don't show OS settings banner if SplitSettings is disabled

SplitSettings is on-by-default. However, if we have to turn it off, we
don't want the combined settings page to show the promo banner linking
to OS settings (which won't exist).

Test: Run chrome --disable-features=SplitSettings with an empty user
data directory. Open settings. No promo banner shown.

Bug: 1000307
Change-Id: If87d7cf3049353a2a1a322c042adb4bfa886f462
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1785478
Auto-Submit: James Cook <jamescook@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693424}

chrome/browser/resources/settings/basic_page/basic_page.js

@@ -236,6 +236,11 @@ Polymer({
     if (!this.prefs || !this.currentRoute_) {
       return;
     }
+    // Don't show the banner when SplitSettings is disabled (and hence this page
+    // is already showing OS settings).
+    if (loadTimeData.getBoolean('showOSSettings')) {
+      return false;
+    }
     const showPref = /** @type {boolean} */ (
         this.getPref('settings.cros.show_os_banner').value);