Document invariants in SearchGeolocationDisclosureTabHelper.

TemplateURLService::GetDefaultSearchProvider() will return nullptr when
the `DefaultSearchProviderEnabled` policy is set to `false`.

SearchGeolocationDisclosureTabHelper::MaybeShowDisclosureForValidUrl()
can, however, assume that the return value of GetDefaultSearchProvider()
will be non-null, because the former method will only ever be called if
a DSE is enabled. Document this in the code in the form of a DCHECK.

Bug: 1011910
Change-Id: Ie8763e9b0e6e030347eb956125aeea7e25e17458
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1855819
Auto-Submit: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705487}

chrome/browser/android/search_permissions/search_geolocation_disclosure_tab_helper.cc

@@ -160,6 +160,10 @@ void SearchGeolocationDisclosureTabHelper::MaybeShowDisclosureForValidUrl(
       TemplateURLServiceFactory::GetForProfile(GetProfile());
   const TemplateURL* template_url =
       template_url_service->GetDefaultSearchProvider();
+  // ShouldShowDisclosureForNavigation() checked explicitly that the default
+  // search |template_url| was non-null, and ShouldShowDisclosureForAPIAccess()
+  // would have also seen an empty DSE origin if it were.
+  DCHECK(template_url);
   base::string16 search_engine_name = template_url->short_name();
   SearchGeolocationDisclosureInfoBarDelegate::Create(web_contents(), gurl,
                                                      search_engine_name);