Fix OS accessibility settings link to browser appearance settings

When SplitSettings is enabled the route to APPEARANCE doesn't exist,
so clicking the link throws an error. Instead, open the browser setting
in a new browser tab.

Test: Open OS settings > Advanced > Accessibility > Manage
accessibility features, click "Open appearance settings"

Bug: 999354
Change-Id: Ibb1e42dccfef72a42779c330ba189061e398da4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1778815Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692176}

chrome/browser/resources/settings/a11y_page/manage_a11y_page.js

@@ -212,9 +212,16 @@ Polymer({
 
   /** @private */
   onAppearanceTap_: function() {
-    settings.navigateTo(
-        settings.routes.APPEARANCE,
-        /* dynamicParams */ null, /* removeSearch */ true);
+    if (loadTimeData.getBoolean('isOSSettings')) {
+      // Open browser appearance section in a new browser tab.
+      window.open('chrome://settings/appearance');
+    } else {
+      // Open browser appearance in this settings window.
+      // TODO(crbug.com/986596): Remove this when SplitSettings is the default.
+      settings.navigateTo(
+          settings.routes.APPEARANCE,
+          /* dynamicParams */ null, /* removeSearch */ true);
+    }
   },
 
   /** @private */