Add check for whether an opt guide is present before loading hints

Bug: 1016987
Change-Id: Ie998aaf0f8d72a4896486c48e04dad120e9618ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1874818Reviewed-by: Robert Ogden <robertogden@chromium.org>
Commit-Queue: Sophie Chang <sophiechang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708475}

components/previews/content/previews_decider_impl.cc

@@ -365,6 +365,10 @@ PreviewsEligibilityReason PreviewsDeciderImpl::DeterminePreviewEligibility(
 bool PreviewsDeciderImpl::LoadPageHints(
     content::NavigationHandle* navigation_handle) {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
+
+  if (!previews_opt_guide_)
+    return false;
+
   return previews_opt_guide_->MaybeLoadOptimizationHints(navigation_handle,
                                                          base::DoNothing());
 }