RC: Fix invalid access when getting site engagement score in chrome://discards.

There was an invalid access when the site engagement score of a
WebContents that was never navigated was obtained in chrome://discards.
With this CL, we return 0 as the engagement score of never-navigated
WebContents, instead of doing an invalid access.

Creating a new tab or opening a link in a new tab are actions that
create a WebContents a briefly keep it in a non-navigated state.

Bug: 854497
Change-Id: I204509b7c41445ac4b524f9a652f006189e4fcd9
Reviewed-on: https://chromium-review.googlesource.com/1111908Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569700}

chrome/browser/ui/webui/discards/discards_ui.cc

@@ -67,8 +67,13 @@ resource_coordinator::LifecycleUnit* GetLifecycleUnitById(int32_t id) {
 double GetSiteEngagementScore(content::WebContents* contents) {
   // Get the active navigation entry. Restored tabs should always have one.
   auto& controller = contents->GetController();
-  auto* nav_entry =
-      controller.GetEntryAtIndex(controller.GetCurrentEntryIndex());
+  const int current_entry_index = controller.GetCurrentEntryIndex();
+
+  // A WebContents which hasn't navigated yet does not have a NavigationEntry.
+  if (current_entry_index == -1)
+    return 0;
+
+  auto* nav_entry = controller.GetEntryAtIndex(current_entry_index);
   DCHECK(nav_entry);
 
   auto* engagement_svc = SiteEngagementService::Get(