chrome://management: back button is a real back button

Instead of always getting redirected to 'chrome://settings/help', the
back button now goes back in the tab's history. If the tab has no
history, it falls back to the old behavior.

Bug: 959258
Change-Id: I32c6700648839b23879c48dd69878fa74ef42986
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1606190Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659148}

chrome/browser/resources/management/management_ui.js

@@ -240,7 +240,11 @@ Polymer({
 
   /** @private */
   onTapBack_() {
-    window.location.href = `chrome://settings/help`;
+    if (history.length > 1) {
+      history.back();
+    } else {
+      window.location.href = 'chrome://settings/help';
+    }
   },
 
   /** @private */