Settings Router Refactor: Update subpage-back button to use history back sometimes.

chrome/browser/resources/settings/settings_page/settings_subpage.js

Index: chrome/browser/resources/settings/settings_page/settings_subpage.js
diff --git a/chrome/browser/resources/settings/settings_page/settings_subpage.js b/chrome/browser/resources/settings/settings_page/settings_subpage.js
index eaa4874cc6186bbd8346c75c05b5d40ba30e0ce2..e7b171a1cc96cfe37bdb379e6adc9fae39057587 100644
--- a/chrome/browser/resources/settings/settings_page/settings_subpage.js
+++ b/chrome/browser/resources/settings/settings_page/settings_subpage.js
@@ -43,7 +43,15 @@ Polymer({
 
   /** @private */
   onTapBack_: function() {
-    settings.navigateTo(assert(settings.getCurrentRoute().parent));
+    var previousRoute =
+        window.history.state &&
+        assert(settings.getRouteForPath(
+            /** @type {string} */ (window.history.state)));
+
+    if (previousRoute && previousRoute.contains(settings.getCurrentRoute()))
+      window.history.back();
+    else
+      settings.navigateTo(assert(settings.getCurrentRoute().parent));
   },
 
   /** @private */