Check for null history service before building DomainDiversityReporter

This CL checks if the |history_service| is valid before it is used to
build a DomainDiversityReporter. This is a guard against unexpected
crashes in DomainDiversityReporter which always requires a valid
history service.

Bug: 1161727
Change-Id: Ie946abe755f7a32bf7b7173e3b146326459edcf5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2611668Reviewed-by: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Mingjing Zhang <mjzhang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841700}

chrome/browser/history/domain_diversity_reporter_factory.cc

@@ -39,6 +39,10 @@ std::unique_ptr<KeyedService> DomainDiversityReporterFactory::BuildInstanceFor(
       HistoryServiceFactory::GetForProfile(profile,
                                            ServiceAccessType::EXPLICIT_ACCESS);
 
+  // Only build DomainDiversityReporter service with a valid |history_service|.
+  if (!history_service)
+    return nullptr;
+
   return std::make_unique<DomainDiversityReporter>(
       history_service, profile->GetPrefs(), base::DefaultClock::GetInstance());
 }

components/history/metrics/domain_diversity_reporter.cc

@@ -31,6 +31,8 @@ DomainDiversityReporter::DomainDiversityReporter(
       clock_(clock),
       history_service_observer_(this) {
   DCHECK_NE(prefs_, nullptr);
+  DCHECK_NE(history_service_, nullptr);
+
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
 
   base::SequencedTaskRunnerHandle::Get()->PostTask(

ios/chrome/browser/history/domain_diversity_reporter_factory.mm

@@ -54,6 +54,10 @@ DomainDiversityReporterFactory::BuildServiceInstanceFor(
       ios::HistoryServiceFactory::GetForBrowserState(
           chrome_browser_state, ServiceAccessType::EXPLICIT_ACCESS);
 
+  // Only build DomainDiversityReporter service with a valid |history_service|.
+  if (!history_service)
+    return nullptr;
+
   return std::make_unique<DomainDiversityReporter>(
       history_service, chrome_browser_state->GetPrefs(),
       base::DefaultClock::GetInstance());