Fix missing WeakPtr check in PreconnectManager

https://chromium-review.googlesource.com/c/chromium/src/+/2295780
added UKM around prefetch accuracy stats, which involved an added line
in PreconnectManager::TryToLaunchPreresolveJobs() that did not check
the WeakPtr before using it. This fixes that.

Change-Id: I3cdf84d0f84fb9cc59811817efb1bdd42d15cf38
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2309029Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790375}

chrome/browser/predictors/preconnect_manager.cc

@@ -231,7 +231,8 @@ void PreconnectManager::TryToLaunchPreresolveJobs() {
                          weak_factory_.GetWeakPtr(), job_id));
       if (info) {
         ++info->inflight_count;
-        delegate_->PreconnectInitiated(info->url, job->url);
+        if (delegate_)
+          delegate_->PreconnectInitiated(info->url, job->url);
       }
       ++inflight_preresolves_count_;
     } else {