Ensure that all members of LocalSiteCharWCObserver are initialized

Follow up on https://chromium-review.googlesource.com/c/chromium/src/+/1131556

Bug: 773382
Change-Id: Idddda163085d58aa4a351ae528544dd9968daaf1
Reviewed-on: https://chromium-review.googlesource.com/1132107Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573955}

chrome/browser/resource_coordinator/local_site_characteristics_webcontents_observer.h

@@ -92,7 +92,7 @@ class LocalSiteCharacteristicsWebContentsObserver
   bool first_time_title_set_ = false;
 
   // The PageSignalReceiver observed by this instance.
-  PageSignalReceiver* page_signal_receiver_;
+  PageSignalReceiver* page_signal_receiver_ = nullptr;
 
   SEQUENCE_CHECKER(sequence_checker_);