Use unique_ptr in PrerenderManager::old_web_contents_list_.

This CL is a refactor with no intended behavior change.

Bug: 832879
Change-Id: I4c86d936907595a631fb9e603c55af90292ff724
Reviewed-on: https://chromium-review.googlesource.com/1044892Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556267}

chrome/browser/prerender/prerender_manager.cc

@@ -1095,11 +1095,6 @@ bool PrerenderManager::DoesRateLimitAllowPrerender(Origin origin) const {
 }
 
 void PrerenderManager::DeleteOldWebContents() {
-  for (WebContents* web_contents : old_web_contents_list_) {
-    // TODO(dominich): should we use Instant Unload Handler here?
-    // Or should |old_web_contents_list_| contain unique_ptrs?
-    delete web_contents;
-  }
   old_web_contents_list_.clear();
 }
 
@@ -1149,7 +1144,7 @@ void PrerenderManager::CleanUpOldNavigations(
 void PrerenderManager::ScheduleDeleteOldWebContents(
     std::unique_ptr<WebContents> tab,
     OnCloseWebContentsDeleter* deleter) {
-  old_web_contents_list_.push_back(tab.release());
+  old_web_contents_list_.push_back(std::move(tab));
   PostCleanupTask();
 
   if (!deleter)

chrome/browser/prerender/prerender_manager.h

@@ -576,7 +576,7 @@ class PrerenderManager : public content::NotificationObserver,
   // Track time of last prerender to limit prerender spam.
   base::TimeTicks last_prerender_start_time_;
 
-  std::vector<content::WebContents*> old_web_contents_list_;
+  std::vector<std::unique_ptr<content::WebContents>> old_web_contents_list_;
 
   std::vector<std::unique_ptr<OnCloseWebContentsDeleter>>
       on_close_web_contents_deleters_;