[CrOS settings] Add DCHECK when opening browser settings page

With most of routes shared between ChromeOS and browser renamed except
for Sync routes, a DCHECK can be used when opening browser settings
pages to that the |sub_page| is not an OS-specific setting.

Bug: 990169
Change-Id: I84ca6c5584d405d3a2deed2d61878c70d3744c54
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2138115
Commit-Queue: Nnamdi Theodore Johnson-kanu <tjohnsonkanu@google.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758529}

chrome/browser/ui/chrome_pages.cc

@@ -386,14 +386,9 @@ void ShowSettingsSubPage(Browser* browser, const std::string& sub_page) {
 void ShowSettingsSubPageForProfile(Profile* profile,
                                    const std::string& sub_page) {
 #if defined(OS_CHROMEOS)
-  SettingsWindowManager* settings = SettingsWindowManager::GetInstance();
-  // TODO(jamescook): When SplitSettings is close to shipping, change this to
-  // a DCHECK that the |sub_page| is not an OS-specific setting.
-  if (chrome::IsOSSettingsSubPage(sub_page)) {
-    settings->ShowOSSettings(profile, sub_page);
-    return;
-  }
-  // Fall through and open browser settings in a tab.
+  // OS settings sub-pages are handled else where and should never be
+  // encountered here.
+  DCHECK(!chrome::IsOSSettingsSubPage(sub_page)) << sub_page;
 #endif
   Browser* browser = chrome::FindTabbedBrowser(profile, false);
   if (!browser)